Business Reasons
Core books sell better than supplements. It's also better to upgrade a healthy product line (which 3e seems to be) rather than waiting for it to whither and shrink its market share (like 2e did)
Design Reasons
3e is eight years old, and those eight years have produced more innovation and evolution in RPG design than the twenty years preceding them. When 2e came out, the big design leap from 1e was the point buy system Champions. When 3e came out, the big design leaps were games like Ars Magica, Amber Diceless and Vampire: the Masquerade.
When 3e came out, The Forge and the entire indie gaming scene essentially didn't exist. There was essentially no understanding of, or even an attempt to develop a coherent example of, RPG Theory.
That's not to say 4e should look like Dogs in the Vineyard or Burning Empires. It is, however, to say that 4e should acknowledge and consider the existence of those games, the fact that RPGs have gone in more new directions since its release than in all the years prior to it combined, and the fact that purpose-driven design in an RPG is now both possible and desirable.
In the d20 sphere alone, games like True20, Mutants and Masterminds and Star Wars Saga Edition are leaps and bounds ahead of the current version of D&D in terms of clear and purpose-driven design, speed of play, and prep time.
Significant concerns have been raised by fans of prior editions about the complexity of the rules - the fact that the most successful and critically acclaimed d20-derivatives all pare down those rules in one way or another backs up their concerns, and perhaps even the ways they want to address them. Many of the d20-derived games are closer to what these fans want and at least as satisfying and appealing to new players, and this is a viable area for improvement.
Mike Mearls, who seems likely to be the lead designer on 4e considering his prominence in the design community, has raised other, equally significant concerns, mostly regarding what he calls user interface issues. Those issues really weren't considered in game design circles prior to 3e's release, and were only beginning to be understood between its release and 3.5's. Addressing them is a viable area for improvement.
Finally, D&D faces significant competition as an IP for perhaps the first time in its existence. World of Warcraft is the first Western-produced fantasy gaming IP to eclipse D&D in both its known playerbase and cultural footprint. D&D may or may not be able to compete in both respects - but it obviously isn't doing so now, and there may be ways for it to do so. This, too, is a viable area for improvement.
