I was heavily involved in Gleemax from the start (I ran the featured news blog for 26 weeks on the Gleemax blogs and was involved in just about everything involving Gleemax) so I may be able to shed some light on the subject.
Gleemax was intended to be a "MySpace for Gamers" that included blog and forum functions. It was also supposed to include a game portal to play games such as Axis and Allies and RoboRally online as well as including original games, such as Uncivilized: The Goblin Game.
The first glimpses of Gleemax were through a viral marketing campaign and a new section of the WotC forums. That's where I came in.
This part is speculation on my part, but I believe that Gleemax and D&DI were meant to go hand in hand as certain elements of the planned tools for D&DI were meant to be accessible via Gleemax and vice versa, such as player profiles and the Game Table.
WotC also put together a "Gleemax Advisory Board" which included two winners of the Ultimate Gamer Contest and several industry luminaries, though the board only ever had one meeting, shortly after GenCon 2007 and a few of the industry folks appointed to the board, such as Monte Cook, didn't attend.
At any rate, not too long after the Gleemax forums popped up, the Gleemax Blogs Alpha Site came along. The first of several mistakes was at this stage of the game because the developers chose to run a public Alpha, which meant the blog functions were far from complete and extraordinarily glitchy.
Still, Gleemax Alpha did feature several WotC developer blogs, which were something of a secondary source of 4th Edition news and tidbits, prior to 4E's release. Of the various game companies invited to use Gleemax, only Chris Pramas of Green Ronin ever posted with anything approaching regularity.
The old WotC forums were also moved on to the Gleemax server at this time and a Volunteer Community Lead program was initiated by the community team of the time (Gamer Zer0 and Solice).
Gamer Zer0 also began running a podcast, Gamer Radio Zer0, which eventually evolved into a video podcast. As with Gleemax, GRZ's original intent was to cover the entire industry, but it evolved into an advertising vehicle for WotC.
Much of the ill will toward Gamer Zer0 came about due to a "out with the old, in with the new" philosophy. Community initiatives predating Gleemax, such as the community magazine Knowledge Arcana and the online convention known as UnCon were given the axe, with nothing put forth to replace them. Eventually, the VCL program took it upon themselves to reinstate those initiatives themselves, though in the case of KA, the effort eventually backfired.
The Game Portal managed to get online, powered by
Game Table Online but it was too little too late as the plug was pulled on Gleemax shortly thereafter, with the games hosted on the Gleemax Games Portal moving to GTO.
Gamer Zer0 was let go just a few days after GenCon 2008. Randy Buehler, who was essentially in charge of the Gleemax project, was moved to D&DI before being let go in the next round of layoffs, along with Solice (the better liked member of the community management team).
Much of the content and organization for the Gleemax Blogs and Gleemax forums came from non-WotC employees, the first members of the Volunteer Community Lead program, of which I was a part. Most of us abandoned ship when the writing on the wall became too obvious to ignore.
As to why the plug was pulled on Gleemax, Greg Leeds from Hasbro came in as head of WotC sometime after Gleemax Alpha went live. It was around this time that focus shifted to getting D&DI up and running, with Gleemax taking a back seat and eventually getting shut down altogether.
As for why the forums are still "forums.gleemax.com" all of WotC's various online endeavors (Gleemax, forums, D&DI, and, I believe, Magic Online) converted to a single login system during Gleemax and that aspect of the software remains. They'll need to move everything (again) in order to switch it back to "forums.wizards.com" or whatever.
Hope that sheds some light on things. Please keep in mind that a) I'm not a WotC employee and never have been and b) I'm no longer a VCL either (I was one of the last of the original Gleemax group to abandon ship, just a month ago).