Because by and large, third party titles don't really thrive on Nintendo platforms. Ports is what the Wii-U had at launch too. Nobody honestly cared much, because those games aren't what people are buying Nintendo hardware for.
Maybe if it was the only console in the market, then you'd have a point.
But with very popular, established consoles already on the market that offer all of the third party titles from day one, many people buy Nintendo consoles just to play Nintendo games. That's not going to change just because they are on an equal power level, porting a game over to a different platform still takes a lot of money and work.
Man, do we have to do this again? What big 3rd party games did Wii get in all of the years it was on top of the sales? Did it have AssCreed? Final Fantasy? Mass Effect? Dragon Age? Street Fighter? Fallout? Skyrim? It had almost nothing of the big third party IPs, and most likely due to technical reasons.
And yes, ports happened with Wii U, let's see, Mass Effect 3, fullprice, with EA releasing the Trilogy pack cheaper everywhere else. Rayman, which even was delayed for "parity", ironically sold good on Wii U, Deus Ex and NfS, i'd agree with you, cause these were good ports, yet Deus Ex was already an ancient game and budget priced everywhere else. So many games with some gimped downsides, missing DLC, missing online stuff, missing support, etc. I'd think that only Ubisoft did actually try, but one 3rd pub alone was not enough.
Yet the Wii U is known for it's strange hardware, strange tools, and missing engine support.
Christ, no sane Nintendo fan is like "not buying a game because it's from EA", and if Nintendo provides a comfortable to work with enviroment, i'd expect ports of the same quality as the other consoles get, and at least a moderate improvement of sales.