[ckessel]

On top of what others have said, by checking the flop you're also essentially admitting you don't have a diamond. By betting the flop, people with a low diamond flush (say a low pocket pair) are likely to fold. If you check the flop, then bet a blank on the turn, you're going to get a caller who has a low diamond and/or a small pair because you're flop check basically told them you either had nothing, or have something that'll be beaten by any flush.

If you bet the flop you stand a decent chance of getting rid of some hands that could out draw you.