5mg seems to stem from Dr Pathomvanich's talks. Anecdotal but this guy claims to have lowered his finasteride usage because it worked so well for him; https://www.reddit.com/r/tressless/comments/3udt0j/oral_minoxidil_pills/. I think western practitioners have been pretty against oral minoxidil until very, very recently, so perhaps that's why they prefer to give the lower doses. Let's not forget, it's been touted as the devil's drug many a time on these forums. Like anything, people need to be speaking to their docs/derms about this stuff.