I was on the BYU cross country team back in the day. Some of the very talented RMs got home and simply didn't have the burning desire they had pre-mission and instead got married or wanted to focus on studies i.e. priorities changed. A few got injured and never came back.

It didn't help that there weren't very many "full ride" scholarships. It was a de facto part-time job equivalent with half scholarships, so you needed to love the sport to stick with it. The training regimen was 60+ miles per week, so it was a time/energy suck if you have competing priorities.