A valid reason if you buy into the bias, sure. Anything can be a valid reason. "I don't want to" is a valid reason for anything, it is also heavily dependent on bias.

The statement "Third parties have a good reason to avoid Nintendo" is a steaming pile of confirmation bias.

 If I'm not mistaken, Nintendo's not exactly too savy on DLC to begin with. Something tells me there's a 2-way issue there. As for the ports, clearly several third party devs have pushed Wii-U passed PS360, but at the end of the day, let's face it, it's a hardware issue there. The system is not very powerful to begin with, and it's severely gimped by the gamepad. You can't blame devs for not trying to conjure up magical elves that somehow make the Wii-U more than it is. Even Nintendo's own first party can't make Wii-U games look phenomenal, unless they feature cartoony, very undetailed, graphics. Point is, you're never going to see anything on Wii-U even come close to the graphical fidelity of a title like inFamous: SS, and if people aren't gonna buy the games anyway, why should third party devs even try?

"But I don't want to, isn't a valid reason. Third parties have very little reason to develope for the wiiu. Nintendo has kind of alienated themselves from devs when they decided to say they didn't need them with the wii generation. Then there's also the fact the wiiu simply doesn't tend to have high sales with almost any game. It's simply not worth the risk to invest into it. 

Yep.  Why waste time and money learning to optimize a console when then the 360/ps3 are completily mapped out already and the PS4/One are vastly stronger and easier to program for.  Then throw in the fact that the game attatch ratio is terrible on the Wii U and you see a situation where it just makes no sense to make games for it.  

And then the market changed. And Nintendo hasn't adjusted to it yet. ^_-

I don't think that's on Nintendo. You could call Nintendo's handhelds just as eccentric as their home consoles. After all, they are all designed by the very same company.

However, third parties do find success on Nintendo handhelds on a more regular basis and the Nintendo handheld had the best third party software sales in every single generation (compared to its respective competitors). Why do you think that is?

Placing the blame on third party devs only is unreasonable at best. Nintendo is the one making deals with these developers to bring content on their platforms. It's Nintendo in the end that has the responsibility to entice developers to create compelling content for their consoles. The fact is third parties don't want to create special versions of their games for a gimped console. Nintendo do things their way, very stubbornly. The Wii managed to be a hit for a certain type of games and crowd. But looking at Nintendo's performance since the N64, the Wii U is pretty much in line with how things have been for them in the market. When the casuals aren't there, the console sells mostly to Nintendo fans. The sales are pretty much speaking for themselves: In-line with the Gamecube and the N64.

Now you bring up the handheld market in the mix. A different market altogether, you have to admit. And one that Nintendo has always been the leader. Making games for the DS and 3DS isn't as expensive and doesn't involve nearly as much risk as making a high budget game for a console that doesn't sell.

"But... but... The Wii??" The Wii was mostly selling casual games. When third party devs made games that catered to the "hardcore" audience, sales were on most accounts disappointing, barely breaking even with the devs investment. It's not very difficult to see why devs would prefer to make their games on system where porting from one an other costs little and brings the most revenue in the end. The risk just isn't as high on the other home consoles compared to Nintendo's. Because that's where the actual crowd they make content for actually buy the games they make. Sony and Microsoft also help those devs by putting ads on their online services. Making sure people hear and know about the actual games...

I think it's really laughable when people blame anyone other than Nintendo for the mess the company is in right now. Developers go where there's money to be made. Totally normal. Figures from the last few generations show how difficult it is for them to make money on Nintendo's home consoles. To deny this, and make excuses like "they're just releasing lazy ports", only tells me that some of you don't want to face the possibility that Nintendo may not be as Godly as you see them.

Likewise, to completely dismiss "lazy ports" as an issue shows a reluctance to face the possibility that devs are responsible for their own revenue.