1. The Wii U uses a different architecture than other consoles, and most modern PCs. The Wii U is based on IBM's power pc. It's not necessarily more difficult, but since most third party games run on x86 architecture natively, it means extra work to port.

2. The Wii U has less ram than either of the other two systems. This means textures and such have to be compressed further than they would on other systems.

3. The Wii U is simply less powerful. This means certain effects have to be altered or taken out completely. Probably not for Elliot Quest, but in general...

As for Elliot Quest, it's not a graphically complex game, but it likely has a small budget and a small team, which makes altering the game to run on the Wii U might be challenging. They also didn't say explicitly it was a technical issue. There are also licensing issues and other red tape with the eShop. All of the console shops tend to be more restrictive than Steam.

Also many devs are never in contact with that architecture.

Lets say I am a builder. I am used to building with stone on concrete. power pc comes and says build that stone house on sand please. He could do it with research and more work but it's not what he/she is used to

Extra effort = more difficult, which means it's not worth it especially when sales are low

I still find it funny.. everyone did say its as easy to make games on it as it was for 360 but yet 360 gets more game still..

The difficulty may stem from how the venture causes "lack of focus" in developers as they work to port a game. While they are porting, thoughts on how unlikely it may be for them to recoup porting costs overtake their thoughts and that leads to mistakes, lack of concentration and the occasional sleep deprivation.

Different architecture, and weaker hardware.

Its not hard to develop for. Its just not worth it.

While I'm pretty sure that by now everyone understands that the Wii U is more "difficult" for 3rd parties to develop for due to different architecture and hardware limitations compared to X1/PS4/PC, my question is why the hell is it so difficult for NINTENDO to develop games for the Wii U.

I mean seriously, none of the games they've put out on the Wii U thus far are of the 100+ hour long or extremely hardware-taxing stuff we see on the other console and PC. NSMB Wii U and SM3D World are essentially HD ports of the handheld games. MK8 and SSBU aren't exactly GTAV or Uncharted 4 either. And Splatoon looks like it was cooked up in about 6 months. So why the hell does it take Nintendo so long to develop (relatively) simplistic games for underpowered hardware?

Wow, you understand basically nothing about anything if this is what you actually think.

since it is an IBM architecture like what mentioned above. even though it is the same as 360, it became hard to port due to weak hardware as also stated. and nintendo couldn't collaborate to bring any AA/AAA games to Wii U to help their hardware sales.

On why Nintendo couldn't help 3rd party and themselves to come up with a ton of AA/AAA games, this is due to the size of their employees for development. hence the dev cycle is longer than expected as it is their 1st time to develop HD games. they have collaboration with 3rd party like Atlus, TK, etc. but i think it is not primarily targeted to the western audience. which we hope Nintendo should do on their next HC.