Because you can't. They're reserved for c-diff infections and there isn't much risk data on use cases outside of that such that regular providers won't give it the OK.

This was 20 years ago. While not brand new at the time, it wasn't very common.

A lot of work on FMT has only taken place in the last decade. I don't think there was even a randomized trial for it as a treatment method until the last 10 years.