I think that's also because of how frustrating it is to get a comp game going. I often only get a game after the 5th try or so. Comp MM is not very good UX wise atm.
Also, i don't know why, but some people have this mindset of "this is beta, why should i play comp" as in: they think comp is real serious business.
Also part 2: the biggest portion of a playerbase normally consists of casuals. Not that that's a bad thing.