From what I understand, and adding my own speculation, I think things went down like this:
1999 - Hasbro buys WotC. Due in part to a clause in the acquisition deal that allowed a post-acquisition price adjustment based on non-Magic CGGs, Wizards' product lines report to Hasbro individually, rather than just one big company.
2000 - WotC releases OGL and 3.0 SRD. Rising tide lifts all boats. Atari buys Hasbro Interactive, including the digital rights to most, if not all, Hasbro properties.
2001 - Adkinson leaves WotC.
2002 - Market saturation starts deflating the bubble, encouraging WotC to release 3.5 two years early. Vince Calouri resigns as head of WotC (for unrelated reasons). Replaced by Chuck Heubner.
2003 - 3.5 comes out. It's OGL, but changes in the rules screw many 3pps. Bubble not helped. Around this time Heubner feuds with Hasbro's Boys Toys division head, Brian Goldner, over who gets to make CGGs at Hasbro. Heubner more or less wins.
2004 - Heubner leaves and is replaced by Loren Greenwood. Greenwood had been against Goldner in the CGG fight.
2005 - Hasbro introduces new internal policy: Core Brands and Non-Core Brands. Core Brands make over $50 million a year, with potential to get to $100 million. Non-Core Brands don't. Because Core Brands are moneymakers, they get more funding, more support from the company. Non-Core Brands don't. Because each product line reports individually to Hasbro, D&D doesn't get any kind of boost from Magic. WotC makes a deal with Atari to buy back many of the digital rights on their IP, but Atari retains D&D for a planned 10 more years.
2006 - Brian Goldner becomes COO of Hasbro. At this point, D&D is making $25, maybe 30 million. Not enough to get Core Brand status (unlike Magic, which easily clears the bar). 3.5 has been out for 3 years. It's generating revenue, but not growth for WotC D&D. Magic is a huge money maker and safe, but with Greenwood-Goldner relations suspect, D&D can't afford to rest on its laurels. We can speculate that simply another revision and release of the core three books would not get them in the sweet spot, as the D&D team instead made an ambitious pitch to Hasbro. D&D was a decent money-maker worldwide -- certainly there were millions and millions of fans. Meanwhile, WoW and other MMORPGs were making huge bank, with folks paying good money for monthly subscriptions so that they could essentially play computerized D&D. Because Atari had the rights, Wizards couldn't simply tap into that market with a MMO, so they came up with a different plan -- DDI. An suite of online tools that while it didn't recreate the MMORPG experience, it could connect D&D to the internet in a way no RPG had been, and they would reap great profits. Plus they would look to MMORPGs, CCGs, and Eurogames for inspiration; these were and are three highly popular kinds of games. Hasbro gave the go-ahead.
2007 - 4e announced. Both game and digital tools are in construction. DDI is not coming along so well. Of course, with a new edition announced and in construction, there's precious little new and bestselling material, so sales aren't great. They make the decision to not go with the old OGL, and the new one they make is restrictive. I think of a number of reasons to go this way, even if they didn't think it was a mistake to release 3.x that way. One reason is that they couldn't know if they could catch lightning in a bottle again, or if they could, would it just be another bubble? And would it even take them to the promised land of $50 million? Quality also seemed to take a dive by 2006. They wanted to make this edition really integrated with DDI; would that be possible with 3pps? Would 3pps bleed off some of that needed revenue?
2008 - Greenwood fired in January, replaced by Greg Leeds (current CEO of WotC). 3pps start backing away from 4e's OGL. 4e books released in June. Melissa Batten, DDI developer, killed in August.
2009 - PDFs are pulled. Rob Heinsoo let go. Pathfinder released. Character Builder goes on-line.
2010 - Essentials developed and released. Rules Compendium released. Andy Collins let go.
2011 - Bill Slavicsek let go. At this point the leaders behind the 4e project are almost all gone. Goldner is still CEO of Hasbro, Leeds of WotC, the Core Brand strategy is still in place, and while DDI is at least a modest success, the online monster builder and VT are released, both in Beta. MB goes full-fledged, but VT stalls. DDI has fallen far short of what was planned. 4e may be a perfectly decent money-maker, but even liberal estimates make it far short of its original goal of at least $50 million. Also, Rich Baker is let go. But the people responsible for that pitch are gone. But, Design work begins on the next edition. Hasbro regains D&D video game rights. The board games are selling well.
2012 - 5e announced.
I've said this before, but 3.x made enough money that they thought they had a shot at $50 million, and 4e made enough money that the far from being mothballed because they couldn't reach $50 million, the new edition was approved and announced with a full media blitz. I find it hard to worry about D&D as a brand as this particular stage. They've got the board games, the video game rights, novels, and now the actual game itself. I don't know if the line as a whole can reach $50 million, but I suspect it will make more than enough to keep it off the "mothball" list.