a) They thought he’d miss. Wedge missed his shot and he was a much more skilled pilot than Luke.

b) For most of his approach he had Darth Vader on his tail. They were probably expecting Luke’s X-Wing to be blown up before he could even take a shot.

Achilles did it first.

Vader hated the moff that commanded it so much that he didn’t do the sensible thing and post up above where the people shot at.

To be fair, we all cheered a little when he choked that bitch out.