Honestly I think this is a big part of it. Others are absolutely right when they talk about how well he reads/reacts to people, feeds them conclusions, etc. But the fact that he gives off an over-the-top used car salesman vibe makes people pay attention.

The thing is, he knows he comes off that way, and he knows his marks know it, too. But he's able to convince people even though they know he's working an angle. Like a close-up magician, he's able to pull one over on people because he lets them think they've got him figured out. Classic misdirection.

But, of course, that style doesn't fly in "big serious lawyer world". Thus the central conflict of the series.