I was thinking about this since some monsters from the 3rd generation did not return in monster hunter 4 while some, such as the jinouga, jaggi, did.

In tri,3rd,ultimate they have the same areas whilst misty peaks was available at the start of 3rd and high rank for ultimate.

putting all this together, yukumo, moga village are possibly in the same country or region in the monster hunter world, while in monster hunter 4 you hunt in newer areas and travel to different villages or locations in a caravan.

The only i could think why capcom did not include some monster from the 3rd generation(although you can trade for their parts, such as the urgaan)and bringing back some older monster from previous generations is that the country or region you are set in is near where the old generation are set in.

That's all i could think about this.

My take in it is space, they can only put so much into the new game. Priority would be anything new, then fan favorites which is tied with anything essential.

And it would also depend how well they can put a new spin into anything old they put back.

Not to mention that if something was already there in the previous game then putting it on the game that comes right after it would be viewed as a sign of laziness and lack of innovation.

2633582
good point about the space since they did include new subspecies for some of the older monsters.