Miller has said repeatedly why he became a conservative. He just saw the left becoming increasingly loopy and out of touch.
The capper for him, according to him, was when he repeatedly heard lefties he had respected refer to Bush as ‘Hitler’. To Miller, this struck him as being not just ridiculous, but offensive. The first couple of times, he let it go. But he said he kept hearing it, or other equally over-the-top assertions, and it made him question whether or not he wanted to be associated with them.
Bear in mind that Miller runs around in Hollywood circles, where the most outrageous extreme lefties live. Hell, hanging around Michael Moore and Alec Baldwin for a few years would turn Hillary Clinton into a Republican.