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This seems to be a bit of an ongoing mystery. One theory, which sounds a little paranoid, is that "the Feds" put the screws on whoever was behind TrueCrypt (it's open source, and the developers are quite secretive). Much like happened with Lavabit, this theory goes, the federal government insisted on backdoors being installed so they could access encrypted data, and the developers refused. I have seen no evidence to support this theory, but it seems like a plausable explaination.
Another theory is that the devs just got tired of dealing with it and wanted to move onto other projects. Since we can't ask them about it, who knows if this theory has any merit.
The official line was that with the "death" of Windows XP, TrueCrypt is no longer needed. That sounds super fishy to me. It isn't as if everyone turned off their XP machines and left the room - there are still millions of instances of XP running on PCs around the world.