Because I'm strongly interested in both subjects, could very well pursue a career in one of them (or related fields), and there are excellent resources for both in the university system, especially for physics (research opportunities, labs, etc.).
I think the consensus around here is that too many high IQ people go into physics compared to what is socially optimal. Unfortunately my Google-fu is failing me and I can't find the posts/discussions I have in mind. (Anyone want to help me out?) The closest I could find is Paul Christiano's The Value of Theoretical Research.