This comes from the Iwata Asks interview...

''To be honest, it started from the point when we thought that since it will take some time to make all five sports and release them at once, what if we were to release them in the order they're ready. If we were to release them in a package, it would be quite some way down the road before all of them were ready. I do think, however, that there are a lot of benefits to this approach. And this is an online game. If we were to release all five games at once, the customers would divide up into the different games. But if we put them out like an irregular periodical, people can thoroughly enjoy each individual sport.'' - Nintendo's Shimamura

Source: Iwata Asks