Because he acts condescending. In my experience people hate being told that they are wrong at the best of times. Thats why you sometimes get people who even when shown clear evidence contrary to their established opinions on something, will doubt it sceptically.
You can't change peoples minds by appearing arrogant in your own opinion. When he debates the existence of God he comes across as though he thinks the people he is speaking to are childlike simpletons. That's what gets people on the defensive.
I myself am an atheist and I think Dawkins is a dick.