Lets start with what benefit they would get by keeping him. If you can't answer that question, then there is no point to answering why he is let go.
Jeremy Evans isn't that good, but he's better than Perkins. Perkins is just awful.Because currently our 4th big is Jeremy ****ing Evans! We desperately need a back up center.
The answer to OP, it is because he was never intended to come here. His salary is too high, and there is no way to know if he was even interested.