There are plenty of discussions on HN about this, including references to talks and writings by Peter Thiel, if you're interested. You could probably get a pretty good idea into why he did. Two references off the top of my head that would be particularly on topic would be his keynote at the RNC and the National Press Club interview.

RNC keynote: https://www.youtube.com/watch?v=UTJB8AkT1dk

National Press Club interview: https://www.youtube.com/watch?v=ob-LJqPQEJ4