In addition to Nintendo's moneyhatting, it's possible Capcom just wants to associate specific experiences to each sub series.
I predicted they'd have two Monster Hunter lines, one for PS/Xbox and the other for Switch. I made that prediction AFTER World's breakthrough. Nintendo's money alone wouldn't possibly make up for the huge loss in sales. Perhaps Capcom surmised porting up to PS4/5 would be a little jarring in the context of World and expectations from PS5.
Still strange that they not once tested releasing a Nintendo + Playstation MH, not even as late ports!
Monster Hunter has had two separate lines long before World, with the numbered entries and Freedom Unite 1-3 + Generations.
I guess they just want to stick with what has worked for them for so long, in addition to likely some generous moneyhatting from both sides.
That model didn't really work out for them coz it was lopsided for handhelds until World, and it was primarily between PS2 and PSP at a time Monster Hunter was still fresh and experimental (and PS2 required an adapter for online and multiplayer). Excluding World, mainline home console games were all ported to handhelds in some form, and if memory serves me right all of them outperformed the original by a good margin. As a result, the home console line was dropped for nearly an entire generation.
Since the franchise exploded with World in popularity, a lot of people assumed there was no way MH would go exclusive to Nintendo platforms again... and yet here we are; not even 17.3 million and counting was enough to stop a Nintendo exclusivity.