Yes, they seem to be low in sales compared to the success of the Wii but:
1st. 2 millions copies sold for any of those games will generate gigantic profits. Most of them don't cost as much to produce a AAA title.
2nd. Before stoping production, Nintendo needs to carry at least one game in the series on this generation. Why? because you still don't have any proof that a game in that series won't sell. Imagine not developing a Wii Sport Resort for Wii U when it sold 35 millions? that is crazy, you have to have another one at least one more time to see how it goes. You cannot stop developing a million seller game because you feel it won't sell.