It's not like he's paying out of pocket to appear on TV.
He's just cashing the checks Subway pays him to have him appear in commercials. If you don't like him, then vote with your wallet and don't eat at Subway.
He's been around for years now, so I guess Subway's marketing departement is pleased with the response they get.
He also probably appeals more to the "average Joe" than some megastar. He's the textbook definition of average Joe. He's plain (not cute, not ugly), average build (down from being a fatty). Doesn't sing or dance or entertain in any way...