Guys, the reasons are pretty obvious.
For one thing, the Wii has a handy package of first party games which are most of the time guaranteed million sellers. Besides that it has about 46% marketshare so I actually think the amount of million sellers is pretty low.
As for the 360, it has been said already. It was out longer. Without any competition, it managed to be much more popular in PAL than it would have been otherwise. This allowed it to have plenty of million sellers that it might not have had in another case. Additionally, the 360 has a high attach rate. My thinking is that people are buying lots of old games at lower prices, but there could be a bajillion reasons.
15 is actually an impressive amount considering that Sony doesn't have the first party offerings of the Wii (and a userbase less than half as well) and still has 60% as many million sellers as the Wii.
As for having less million sellers than the PSP, there are three things to say about that:
1. PSP software is grossly undertracked
2. There are several titles on the PSP that should be at a million that aren't
3. The PSP has been out for about 4 years now
The 360 was unopposed for a whole year. So while it may not have had hardware dominance over this generation, it still has dominance in million sellers because when it launched, it was the only next-gen console, and it was the only way to play the games of this generation on a big screen and with advanced graphics. It managed to snag a lot of customers who might not have bought the original Xbox.
