He's greedy, that's why. The Bengals knew he'd be a problem when they drafted him yet they picked him anyway. Andre Smith showed throughout his college career that he only cared about himself, he got suspended for the Sugar Bowl and it didn't bother him. Smith is apparently asking for 5 years $40 million, $1.5M more than what Darius Heyard-Bey got at the #7 pick, the Bengals are offering 5 years $33M, that's a lot of double cheeseburgers he could buy to improve on his unbelievable physique shown below.
Just skip to the 40 second mark. This is why Andre Smith hasn't signed. Everyones making fun of his shirtless 40 yard dash.
Also note at the 1:40 minute mark, You can see Andre Smith run with no shorts not, just his briefs.