I'm afraid my source for this is a half-remembered conference talk from someone who I believe worked for the TOR foundation. My best guess technically was that

I'm afraid my source for this is a half-remembered conference talk from someone who I believe worked for the TOR foundation. My best guess technically was that they didn't want to invest R&D effort into the form of Deep Packet Inspection that came out as a result, for a project that could get them bad press or hauled before congress.