I always figured it was fallout from moving to bipedalism. So many structural changes going on - femur, pelvis, spine, pelvic floor, ligaments, muscles, etc. It doesn't seem that outrageous that a regulatory change needed for bipedalism might also affect baculum development. Also, once you're bipedal, the penis is, well, out there. There might have been advantages to not having a bone in a much more exposed penis. And it's just as easy to envision changes in penis anatomy driving changes in social behavior as it is the converse.