Because he had a hundred FBI agents sending him their reports every day. Felt was Deep Throat. You don’t have to sell me on the fact that Haig was a despicable creep. I acknowledge that fact. But he was not Deep Throat. He would have gladly been if there was something in it for him.
For Felt on the other hand it was strictly a matter of revenge. Felt thought he should have been promoted to the Directors job after Hoover died. Instead, Nixon bypassed Felt and hired a new director from outside the Agency. Felt was pissed and got even.
We’d have to really dig down into the details to find out what “Deep Throat” told Woodward/Bernstein and what Felt was in a position to know.
Here’s a place to start, maybe:
https://www.theatlantic.com/magazine/archive/1992/05/deep-throat-an-institutional-analysis/304084/
Felt did exactly what McCabe, Strozk etc did to Trump with the stupid russia thing. Selectively leaked to give the appearance of a sinister crime. There was zero evidence that Nixon did ANY of the things the media claimed. Drove his poll numbers down, mid-terms were coming up and they forced him out. He resigned mainly because of his health.