I'm afraid there's a lot of very good conservatives, including many on this forum, who can't see past the immigration thing. And Trump is mostly right about it. But it's not the only issue. Most of his supporters think it is.
Immigration is a factor, but I suspect the primary factor is simpler. People want a winner. They are desperate for a winner. And they are tired of a leadership that has compromised too easily. Trump presents himself as the man on the white horse who will lead us to victory. Just how, when you don’t have the votes, is a mystery, but Trump is not a details guy.