Right now he is getting paid as a commentator. If he declares, he has to give up that job.
Right now he can effectively get paid to make his opinions known.
If he declares, he will have to spend millions of dollars to buy airtime for campaign ads.
He will also have to spend a large part of his time raising campaign funds.
He will also lose the money he currently makes as reruns of Law and Order that he is in are shown, because those will have to be taken off the air until the election is over. There are likely contractual issues involved in that as well, and he would likely also be preventing some of his friends and coworkers from that show from making money off of the reruns.
There are a lot of good reasons for Fred to wait as long as he can before declaring.
On top of that, I'm disgusted that this campaign is already in full-throttle just a couple of months after mid-terms, and that we've had a major debate a FULL YEAR befor it should have taken place.