Because he went against the then rulers. He was, therefore, a socialist revolutionary who not only threatened their political power but also their (and the Jews's) religion. He did say, 'Render unto Caesar that which is Caesar's', which should really have pacified the Romans, but that did not save him from the powerful religious establishment - and the Romans saw more advantage in appeasing them.
Hmmm. I've just typed that post-haste. I'm amazed, but it seems to (in some way) sum up what I feel about what happened. Open, as ever, to correction, of course. :)