I think that is the true reason. However I don't blame Nintendo for this. They don't make those kind of games. Last Generation those who make these kind of games tried everything they could to lure the people away from the Wii to the HD-Twins. Then when the WiiU released they "tried" to lure the people back, but how should this work with the same games the people already played on the HD-Twins?

The only Third Party which really tried was Ubisoft with ZombiU. But one game isn't enough to turn the table.

I think its simple really.

The WiiU is an inferior piece of hardware. The people who buy a WiiU do so because they want to play Nintendo games. People are not going to buy a multiplat on the WiiU because it will be inferior. People who like multiplat games (COD, AC, BF, etc) are not going to get a WiiU because they know it will have the inferior version of those games. Therefor there is no audience on the WiiU for 3rd party games.

Nintendo likes to control anyone who puts content on their platform. Its kind of why they dont have a great relationship with third party since the 90's. Thats one of the major reasons besides Sony entering the gaming industry as to why they lost the multiplat gamers. If they get competitive they can get some of them back this gen. Most of the gamers havent even jumped ship to current gen and if Nintendo plays ball they can get some legitimate gamers under their belt again.

The problem is that there's no audience for them in the Nintendo consoles. Nintendo accepted and now are moving to look for solutions (NX Fusion).

The gamecube got 3rd party games.  So your point is?

But if Nintendo had 3rd parties + 1st party games, wouldn't then Nintendo have monopoly in this industry? Does third parties really want that?

Devs are not going to port their games regardless of power because there's no audience that will buy their games. It will be just a waste of time. If Nintendo wants those third parties, not only power will be needed, but they must be commitent to build an installbase willing to buy those kind of games.

No. Nintendo's practices were not good and many will better support only PC before releasing their games on Nintendo platforms.

Sorry, I didn't understand what you mean 100%. Could you explain more detailed?

What I mean, is that third parties wouldn't like that to happen. They don't want another NIntendo's monopoly (like NES, SNES era) were they had bad practices. Third parties didn't have good relations with NIntendo, and they better stop supporting consoles and focus solely on PC, than supporting Nintendo platforms.