It's obvious that they want to bring wii u games to switch so they can make some money off of them since they didn't on the wii u but we don't know the rest of their reasoning behind the choice if there's more. Because nintendo doesn't have much of any 3rd party support, this could be easily a case of nintendo really wanted to fill in some of those gaps in between their major releases which is fine I guess. I think it does tie up their studios that could be used on something else.
I think I was very clear about reasons: easy profit, more diverse and stronger lineup of games (filling gaps comes with this point). Its win-win situation in any case.