Probably because Sony is tired of love mods.
Sony and Microsoft have very strict standards on the production and distribution of content on their platform. Microsoft has had the benefit of interacting with the mod community for years thanks to Halo and Flight Simulator, whereas Sony has been very restrictive (LittleBigPlanet and ModNationRacers are exceptions, but that's likely because their creation tools are pretty limited by comparison).
The issue for Sony is that this content, while moderated to meet the content standards (no love mods), won't be certified for technical quality and has unlimited potential to damage the game experience on the platform or the platform itself (somebody could easily post a mod that has enough AI in one small space to freeze the game).
Now, we all know this is crazy, but you have to see it from Sony's eyes; they have a platform that best out Nintendo and SEGA on the strength of their certification program, and historically they've been very cagey about the way they work with independents. It's not that they don't like modding, but they're just not ready to deal with any of the potential problems if people exploit the system.
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