[dishe]

Word on the street is that it has something to do with system cache. If its in the ROM, it gets included in the system cache, and that memory space and system threading gets priority over other parts of the OS.

Think about it from MS's point of view... they want things like phone operations to have priority over installed apps like DivX players, IM apps and any other third party apps.

But this is just a guess. Only MS (or perhaps HTC) would know for sure, and we won't get an official statement from them because they are trying to pretend that custom ROMs don't exist...