Exit nodes would let them decrypt plaintext network traffic coming out of the exit node, but if they controlled some of the entrance and relay nodes, too, the chances of being able to deanonymize traffic increases.

Given that Tor is a US government creation, I'd assume they'd find value in controlling as many different nodes as they can in order to deanonymize traffic. Tor only has/had value to them as a way for government agents to communicate clandestinely in other jurisdictions.