Has nothing to do with his locker room popularity, and everything to do with how the Patriots historically have done business.
With the exception of Brady, the Pat's just don't renew contracts. They got Moss for a 4th round pick from Oakland. Got some great years of productivity out of him, and now ship him off for a 3rd round. It's really all about business, and they had no desire to keep him after next year. They now have 2 picks in each of the first 4 rounds next year, so they are gonna build with some new talent. It's a smart business move, and shouldn't be anything about Moss. Brady really wanted to keep him.
It's also about Billicheck being an unloyal fuck.