Because he has professional and economic reasons for calling himself a Democrat.
His well-paying TV gigs give him added benefits such as guest-speaking across the fruited plain for renumeration and expenses.
So there's good reason he fills a "token" spot on Fox....being a "renegade" Democrat who speaks like a Tea Party conservative is rather an entertaining novelty.
If his TV persona was that of a true Democrat, he'd be just another predictable, boring hack spouting 'Rat talking points.
This is not to say he's not sincere in his views, because I feel he is. But we have to remember that TV punditry is also show biz....and the pundits adopt various roles which are of the most professional and lucrative benefit to them.
Leni