I really don't get why so many people almost only want to play 5s. I would think that 1v1s or 2v2s would be pretty popular but I only see a few people who regularly seem to host them. The thing about smaller games is that your importance on the team is greatly increased - whether your team (or you) wins or not is much more dependent on how you play than it is in 5s. For me, at least, having significant control over the game's direction as a whole is an extremely positive factor. The results are significantly more correlated with a judgment of your skill than on how lucky the team picks were.
And people seem to care significantly about winning, obviously, and don't like it when idiots or bad players on their team drag them down. Yet so many people still want to play 5s where an individual has much less control over their side's direction as a whole.
I've heard a couple people say that one of the reasons some people don't often play smaller games is that they take longer due to choke points etc... but I've never noticed that myself. In my games, at least, often the game is decided before the hour mark and there's little point in continuing and the losing player surrenders, or is persuaded to.