What prevented all those Gameboy games from being ported to SNES/Genesis or GBA games to PS2 or PSP games to PS3/360 or Vita games to PS4?

Ya they did to an extent but GB/GBA/PSP/Vita all have/had a large list of exclusive games.

Nintendo wanting  to sell their hardware and all the fact that most Gameboy games wouldn't look good on SNES or Genesis????????????? 

Coudlnt disagree more.Its simply a mater to market to your audience.Japan, for example, is a market that dosent care that much about graphics, or it dosent resonate to them as it is for the west, so if you price your game right and cater and market it right, you wont have a problem.And as you said, if developers see a potential on the Switch, they will simply develop for it, no matter the scope of the game.There will be some games that will need adapting, like Professor Layton, but other than that, its not a problem.

I sincerely dont see this as an issue, at all.

Again, I never said that the problem was MH becoming Uncharted. THe problem is that they have developed a handheld system, that essentially is a bit more powerful than a Wii U. Third party devs are no longer contained to a handheld for their games, they can just throw multiplatforms out the wazoo. 

Not much...I can agree with that. Yet, the Wii and Wii U Monster Hunters sold really well. And they have an established Playstation fanbase. Even if it just sells 300k, that's potentially 300k switch consoles.  

"But Switch already offers that too, so you can make a game for one system and address home console and handheld preferences at the same time."

Which just makes it easier to port to the competition. But I get what you mean, if a PS4 owner wants a MH on the TV, they might buy a Switch. Unless, that is, they say fairly early on that the next MH is coming to PS4.

Oh, then sure it can happen.You would need to see if it is worth the effort, due to these franchises being mainly nintendo and handheld centric, which PS arent(obviosly).I would imagine that Nintendo would try to keep some of these titles exclusive to them, like MH, but other than that, its possible that they went multiplatform.

SMT 5 is confirmed to be exclusive?

Added A TLDR at the bottom

That answer makes absolutely no sense.

The discussion is about 3rd parties, do you really think that Nintendo has a say in whether 3rd parties released ports of Gameboy games with color on SNES/Genesis or GBA games to PS2?

Even if they could control that, that doesn't explain why PSP & Vita have a large number of exclusive titles.

Oh ok. Well I don't really think that's a fair argument. The PSP was no where near close console and the Vita had a lot of exclusives that relied on portablity. A lot of Vita exclusives are third party answers to Monster Hunter, which was only on 3ds because that system was the most popular. If Monster Hunter was on Vita, who knows maybe i'd be on ps3. It's speculation, I'm not saying it's going to happen 100%

Edit : Let me put it like this. Small developers making titles for Vita that mimmick monster hunter are only on ps vita because they are competing with handheld monster hunter games. If they were a well established series like Monster hunter, they'd be on more consoles. In fact , Toukiden is on PS4 and that's a monster hunter clone.  Gravity Rush also started on Vita and ended up on ps4 because that was a more successful platform. Obviously these comparisons aren't without flaws, which is why i'm speculating.