http://ir.capcom.co.jp/english/news/html/e071204.html

Capcom is pleased to announce that "Monster Hunter Freedom 2" for the PlayStation Portable (PSP®) has shipped over 1.5 million units domestically, making it the first to do so on the platform in Japan (as of December 4, 2007).

　The "Monster Hunter" series is a hunting action game centered on epic battles with giant monsters in the grandeur of the great outdoors. Since the release of the first Monster Hunter title in 2004 for the PlayStation 2, the series has sold a total of 4.8 million units (as of November 30, 2007) and is one of our most valued titles. "Monster Hunter Freedom 2" was released in February, 2007 for the PSP®, with a focus on co-operative play, which has been hardily supported by youths of all ages across the country. Thanks to the favorable response, the title has grown to become the first ever title in Japan to sell over 1.5 million copies on the PSP®.

Cause the PSP versions have been way more successful then the PS2 versions... which would likely only sell worse on the PS3 due to the obviously smaller userbase.

What are you proposing they do? They could keep it on the PSP, that's certainly possible, but the previously announced platform had been the PS3. 

Given that this is a Japanese-centric game (hugely so, as about 90% of the game's sales come from the region), it certainly makes sense to put the game on a system that has 3x the userbase AND a higher attach rate, with nearly 4x the total software, LTD. Again, PS3 was their previous choice. I'd agree that PSP wouldn't be a bad option either... but that wasn't on the table in the first place.  

yea it wasn't nearly as successful on PS2. This seems to be the only game that breaks all the stereotypes of other PS2 games. It's still cruisin along and staying in the top 50 for like half a year now.

MHF games are the definig games for the PSP in Japan ... on the PS2 they where just another RPG game ... the lack of other high profile RPGs made MHF1 and 2 so succesfull in Japan :)

 Yes , but MH games only sell well in Japan , and there the PS3 numbers would be max 2 million untill then ...