Because the antagonist that gets help become part of the protagonist group. =P
In all seriousness, some people have just gotten their morals and worldview so twisted they can't readapt, there is also many that just don't think there's anything to fix or get help for, regardless of their past. Many times protagonist actually tries to reach out only to be stabbed or mocked for it.
I do enjoy a good redemption arch but also enjoy the irredeemable that you see the potential and wish would accept but ultimately falls back. Heart wrenching even.