Jan van Leyden
Adventurer
What about new material for older editions? Not a book, probably, but DDi articles such as "Combat Maneuvers for 1e" article, "Prestige Classes as Kits" for a 2e article, and "Paragon Paths as Prestige Classes" for a 3.5e article (or similiar themes, like new feats, spells, monster conversions, etc.)
Why don't they do that!? That can rake in tons of cash and fans.
Tons of fans? You mean older editions' fans are even more overweight than 4e ones?

I presume that gamers still using 1e for example have long since detached themselves from what WotC is doing. They are used to make up their own stuff and band together using the internet.
3e fans have mostly - a wild guess, I know - changed their allegiance over to Paizo and Pathfinder. A look at these and other forums indicates a very emotional stance with WotC being tagged as Evil. Could they be ensnared by the Evil side? I don't think so.
Another point, especially were pre-3e editions are concerned, is the necessary system mastery. Are there WotC employees who could without undue effort produce good stuff for these systems? Stuff which can stand the test of gamers very much invested in their systems?
Next would be the fragmented audience. Gamers have a precise idea of what has to be in the game. Say you put up an article series about Dragonborn in other D&D worlds. Which of the many different areas of Dragonlance would you use? Is it pre- or post-Greyhawk wars? Why can't I use this article to make a FR Dragonborn for my campaign set before the Times of Trouble?
I frankly don't see a reasonable approach for WotC to handle legacy systems supplying those gamers with new material. Perhaps they could open up the magazines for publishers of clone systems, say letting them publishing a Dragon article and a Dungeon adventure for one of those pseudo-legacy systems each month. This might make the affected gamers somewhat happy without overly taxing WotC resources.