As a prior comment points out, the popularity is mostly concentrated on portable entries -- though that is not exactly the whole story.
For those unfamiliar with the series, Monster Hunter games have a tendency to recycle a lot of their assets: animations, textures, hitboxes, enemy and character models. Not just *within* the game but across games. Some of them have literally been in constant use dating back to the original game.
Creating new assets from scratch would be super costly for them, and PS2-era assets tend to not look so good on stronger hardware, so they tend to prefer lower spec machines to better hide the age.
This why even when Tri chose to forgo portables in its initial release, it wound up on the Wii. Capcom really did not want to release the series in HD if they could avoid it.