Actually it is a 12-team competition, because the initial round doesn't feature any of the big-4 teams. 4 teams will proceed from the initial round, and teams such as India and Pakistan will feature only in the second round. The first round is more like a qualifier tournament, where the main teams doesn't participate. Ideally I would have liked a 24-team competition, with 4 groups of 6 teams each. But then, the current eligibility criteria means that a lot of teams completely consisting of foreigners may also qualify, and that will reduce whatever interest fans have in expanding the world cup.
One main reason is absence of strong cricket teams, that's why we see few teams competing for the T20 and odi worldcups. There are only 5 to 6 cricket teams that are strong enough to compete in every worldcup rest are weak and associate teams. Cricket is old game but still not many countries joined it.