1. Tor is open source. Any backdoor attempts in the source would require careful hiding. Shutting down the Tor Project would just result in someone else picking it up.

2. The US Navy funded (maybe still does) Tor. Tor is useful for western allies & spies

3. Many of the developers live in western nations. While western nations like the US do have intel agencies who are interested in messing with Tor, it doesn't seem to have gotten to the point of shutting down the Tor Project or directly attacking their infrastructure much if at all, especially since Tor benefits the military as well.