The "Moriya line" is rare because none of them give birth to their own type, and no other yukkuri can give birth to their type as a proxy mother (a Reimu impregnated by a Sanae can not give birth to a Suwako, Kanako, or a Sanae; at least, that was previously the story ...). According to faux-fanon comments by the real Komachi, Komachi-types are endangered and any sale of them would be illegal anyway. Mokou- and Kaguya-types are supposed to be rare, and there is no indication they even reproduce. Kourin-types are probably also very uncommon.
Ironically, the most sexually tenacious type - Alices - are still not considered the most common.