What about to compare the 3DS in Japan to the PS2 in Japan?

Your comments about the PSP are noted but they do not answer my question. Frontier and Portable 3rd HD being exempted from the exclusivity agreement only make the case for one less believeable.

So, exactly when was this contract between Nintendo and Capcom signed, and what were its parameters? The agreement must have been made when MH Tri began development for Wii, and one of its parameters must have been the cessation of development for all Monster Hunter titles on non-Nintendo platforms that are not Frontier, Frontier expansions/sequels, or ports of previously released entries. But, Portable 3rd was released on PSP after Tri. Very well, then the cessation of development clause must have applied only to home console entries. But that would not have prevented Capcom from developing future entries for the PSP or Vita. So the agreement must have stipulated that Capcom was allowed to release one more entry for the PSP after the release of Tri, and none after that. And that this game was eligible as a port to a non-Nintendo console even though it was not a "previously" released entry. Oh, and Freedom Unite may be released in the West.

Also, assuming this deal was made in 2008 (after Freedom Unite was released in Japan, because if the deal was made before then it would have added another complication to the contract), then it was set to last at least six years. That is a looooooong time in the video game industry. The simplest explanation is often the right one. The Wii and 3DS were less expensive to develop for and had larger install bases than their competitors. Nintendo is an extremely cooperative partner for Capcom, publishing the series in PAL regions, allowing the use of many of their IP in recent entries, and letting Capcom do whatever it is Sony wouldn't let them do with the online stuff.

Fixed^

To compare the Wii in Japan to the PS2 in Japan is laughable at best. Their situations aren't close to being comparable. Doesn't help your case that third party games sold better on PS3 than they did on Wii. With so many games on the way on PS3 during development (including many notable Capcom titles like DMC and Resident Evil), there was no reason to believe the PS3 wouldn't pick up in sales.

The PSP wasn't relevant in 2004 when the first MH released (you know, before it launched). They couldn't have developed it multiplatform because it was finished months ahead of the PSP release. They ported the 2nd one because the first handheld game was such a surprising success.

Frontier is a MMO and isn't treated as a mainline game. It's neither important or relevant. Frontier the bastard child of the series and it's like considering FF: All the Bravest or Shin Megami Tensei: IMAGINE as main entries in their respective franchises. Portable 3rd HD doesn't really dispute anything because it's not a new game. At least MH3U added new content from MHtri. 

why? He still talks about Bayo2....

MH Tri for Wii is so far the MonHun with the most success in the West. They had their PSP-titles for Japan and the Wii-title worked for the West. Insofar it was a successful strategy. Also there is another thing to consider: PS3-development involved HD, while Wii was SD. That made dev-cost lower. I just think Capcom deemed dev-cost for Wii and PSP as lower than for PS3 and assumed most fans would follow anyways. Sales say this strategy was successful. Probably the sales of a PS3-version wouldn't have been much higher, but it would have been much expensive to develop.