Well, the associate producer didn't "just quit." That happened half-way through the third season, due to a variety of problems Justman was having with the production. Obviously, money would be an issue to whoever was running the series, but the real problem in the third season was the writing, and the new writers brought in for the third season weren't especially good at writing Star Trek
With the benefit of hindsight, it's easy to say that Bob Justman should have been promoted to being the producer, and allowed the rest of the hires to follow from his guidance. Freiberger didn't do a bad job, but the talent he brought with him didn't turn out to be a good fit, including Arthur Singer.