MH4U is the fastest selling MH game ever in the US!

[StarOcean]

While I do like MH, why is it doing so well? The 3 games Ive played of the series seemed to be exactly the same games with identical areas and objectives. Of course, I've only played the PSP games which may be part of the reason (to note: I'm not planning on getting this or any MH game in the future, can't get into it in the way most MH fans do).
Perhaps I misunderstand and MH is similar to the Street Fighter series in the sense they release the game again but with a few added features?