Kotaku: The Monster Hunter series wasn't always the "monster" (heh heh) hit it is currently. Originally released for the PS2, the first game, Monster Hunter, and its sequel, Monster Hunter G, sold less than a million copies each. It wasn't until the game went hand-held to the PSP with Monster Hunter Portable (Monster Hunter Freedom in the west) that Capcom scored their first million seller with the series.
There is, however, something to be said about the fact that the game series helped cement the PSP's popularity in Japan among the previous generation of handhelds (so much so that the PSP continues to outsell the PS Vita) as well as establish the popularity of the 3DS in the current generation of handhelds in Japan. Why does the series garner such popularity mainly in Japan?