I think part of the problem with WotC supporting D20 Modern is that it's a rules system with so many possible lines of interest.
For example, my home D&D campaign takes place in Green Ronin's Freeport setting. Even though the vast majority of WotC's releases aren't specifically tailored to this type of setting, they still contain rules and other material that I could use.
On the other hand, my D20 Modern campaign is a modern-era, Indiana Jones-style game with very low magic. While books such as Critical Locations, the Menace Manual and the Weapons Locker have been of use to me, ones such as Future, Future Tech, Cyberscape, Apocalypse and (had it come to be) Spectaculars have not been of use.
This same difficulty applies to support for Modern in the form of adventures; many people have many different particular interests, making it difficult to support anything specific.
-Nate