Let me explain here for some people a little bit about the history of D&D.
Early on D&D was huge.
Good historical perspetive through that point.
D&D started as an innovative 1:1 scale wargame, that is, each figure represented a single creatures. The first incarnation of this wargame was called Chainmail, and D&D initially used the Chainmail system for it's combat rules. Chainmail was greeted with some enthusiasm, but D&D made a big splash because it was something quite new and different, and was very soon being called a role-playing game. D&D's first competitors came out only a year later, and would have been in development for some time before D&D was released, so may not strictly speaking have been imitators. Traveler is the prime example. But, D&D was first out the gate, and that gave it a cachet that no subsequent game ever had. By the early 80s, D&D was a full-on 'fad,' as close to being mainstream as the hobby ever got. By the end of the 80s, the fad was over. But, the RPG community, while tiny, was not inactive, the 80s saw many excellent and innovative games introduced. None rivaled the bloated corpse of the D&D fad for sheer revenue, but many left it in the dust for innovation and quality. In the early 80s, several companies consolidated games around proprietary 'core systems.' The obvious one is Chaosium, whose 'Basic Roleplaying' was a core system shared by it's successful RuneQuest FRPG and it's licensed Call of C'thulhu RPG, others included Hero System, Interlock and d6. Core systems eventually led to attempts at a 'universal' system, GURPS explicitly aimed for that, though it eventually re-cast itself as 'multi-genre,' and the 1989 version of Hero System was a de-facto universal system, as well. Univeral systems made a big splash, and some hard-core hobbyists were and still are into them, and core systems each had their following, as well. But, none of them were ever wildly successful. (It's important to keep the very modest success of core/universal systems in mind later...)
For the next 10 years, D&D did little more than rest on the laurels of it's 'first RPG' pedigree and had-been-fad status. A second edition came out that was slicker and cleaned up a little, and it jumped on the setting-over-system (style-over-substance) badwagon of the 90s. That trend was started by Battletech (of all things, a tactial /board/game) in the mid/late 80s and continued by Storyteller, which also got a snide role-not-roll division going in the broader community, D&D bearing the brunt of the 'roll playing' snub. At the same time, D&D, and RPGs in general, were pummeled by a new fad, CCGs, courtesy of our friends WotC, which sucked a whole generation of gamers away from them. LARPs also cleaved off from the RPG community, and many new gamers started there, instead of with D&D, as well. Though, even as CCGs drained away the traditional teenage-boy demographic of new RPGer, LARPs and Storytelling were bringing in female gamers in unprecedented numbers (precedented numbers being /really/ small). Later in the 90s, Hero Games, publisher of it's eponymous 'universal' system, and R.Talsorian, publisher of the Interlock 'core' system, teamed up to create Fuzion, a customizeable universal Open Source rpg. It was joined (or perhaps, preceded?) by FUDGE, a univeral Open-Source, /free/ RPG engine. Neither were terribly successful.
D&D, through most of the 90s, in the hands of rather cynical, decidedly non-gamer, management remained a profitable franchise, raking in money from books, novels, and endless supplements and settings (each selling less than the last), until, after a couple of spectacularly failed spin-offs (a CCG rip-off among them) and a catastrophic misstep in the publishing sector, TSR finally went belly-up. It seemed that D&D, long the dinosaur of the industry, was going to finally go extinct.
The 90s closed, however, with D&D 'saved' by it's arguable nemesis: CCG inventor, WotC. Ah, irony.
By 2000, D&D had a new "3rd" Edition (even though Original D&D, BECMI, AD&D, and AD&D 2e were arguably each editions in their own right), and, like Fuzion and FUDGE, it had a core Open-Sytem behind it. d20 had the advantages of an open source system, /and/ the advantage of D&D name recognition. Other RPG companies fell all over themselves creating content for d20, particularly for D&D, rocketing D&D back to a position of titular as well as default (as the only mainstream-recognizable RPG) revenue leadership. 3e also made real changes to the system, 'modernizing' it to a degree. This set off a wave of rejection by old-school fans, but, in the face of the 3rd-party clamor to jump on the D&D name bandwagon it didn't amount to much, though, parodies of early D&D, like the semi-serious, respectful Hackmaster RPG and the irreverent Munchkin cardgame did meet with some success.
If there's a high-water mark for D&D other than it's 80s fad period, it's early 3e, when Open Source d20 was king of the new RPG hill and erstwhile competitors like WWGS faltered and clung to the bandwagon.
Then WotC was acquired by Hasbro, and all that savior-of-D&D goodwill and d20 euphoria started getting squandered. First, to flog extra revenue out of the line, they re-released a 3.5 version of all the core books, this time, /not/ as modest-priced loss-leaders. There was some loud dis-satisfaction with the move, but "3.5" received it's own SRD and was part of the OGL, so 3pps happily went with it, and hold-outs eventually had nothing much to look forward to but new 3.5 material. The 'oughts' remained D&D's decade, though, and it ended with the decade with a remarkable bit of hubris called the GSL...
After WotC released the rising tide raised all RPG publishers, Open Source d20, Hasbro decided they didn't want to share "D&D's" success anymore, and the next edition of the game was linked not to d20 and the OGL, but to a poison-pill GSL that was not truely open-source, but revocable, and also required anyone signing onto it to get /off/ the d20 bandwagon for good. It did not go over well with 3pps and most nervously dithered, while some bifurcated into an OGL and GSL 'house.' The new edition wasn't just a touch of paint like 2e, but a serious re-tooling, like 3e. But, while 3e was a serious re-tooling after 20 years of relative stagnation, 4e was a serious re-tooling after 8 years of fairly steady development. 3e was not regarded as the dinosaur that AD&D had become when 3e was new. So, the backlash against 4e was even more pronounced than against 3e. And, there were all these publishers, wondering if they should go OGL or GSL. Well, the initial vocal rejection of 4e made it obvious there was still a market for d20 D&D - and the OGL couldn't be taken away, while the GSL could be. Not a tough decision. With scads of 3pp support, staying with 3.5 was a lot easier than staying with 2e or 3.0, and the D&D market seriously split.
The anti-4e backlash and success of d20 fed on eachother, rising to the pitch of the 'edition war' in the volatile on-line community. WotC backpeddled from 4e, first by caving to complaints about 4e with a re-done line of 'Essentials' books, and when that did little more than further shrink the 4e market, announced 5e.
5e, so far, is focused on the noise that surrounded 4e - the same sorts of complaints already addressed unsuccessfully in Essentials. It has yet to take a position on the more critical marketing question: to OGL or not to OGL. OGL could mean turning competitors back into partners and sweeping D&D fans back into one 'latest & greatest' d20 family of WotC and 3pp games. Not-OGL could mean 're-capturing' the 'lost revenue' currently being whisked in by those same competitors - it could also mean another round of rejection by fans and catering to that rejection by d20 competitors.