Wow, people really need to chill.
GTA: CW is a flop, I don't really care if it made money or didn't (I would assume it didn't).
It's a flop because it didn't sell well for a GTA game, period. Not that 1.5 million is bad, but it's just not...great.
DISCLAIMER: I'm not being a fanboy with my following statements, they are my opinion based on some facts.
The game flopped on the DS probably because of the style (graphical and gameplay styles) and potentially some other factors (demographics, lots of other games to chose from, lack of marketing, etc.). I say it flopped because it's a GTA game specifically designed for and (seemingly at the time) exclusive to a system and it sold to a very very small percentage of owners of that system.
For the record, Lair, Haze, Soul Calibur Broken Destiny and lots of other Sony exclusive games are also flops, so, don't go there.
The game flopped on the PSP because, to be consistent, the game is selling at roughly the same percentage as the DS game, so, flop. It probably flopped for some slightly different reasons. Obviously the game play and art style reasons are the same. Some of the game play elements just don't work on the PSP that well as they did with the stylus. Also, the game came out long after people were exposed to it on the DS and thus, perhaps people were less willing to buy it for the PSP based on the GTA name alone. So, the PSP game flopped for the similar reasons as the DS version, just swap out "demographics" with "not the best of ports."
Overall though, the game is a flop because no one liked the game not because of a system it was on (I mean seriously? What kind of logic is that? I mean I could see if a system had like 80%+ of a certain demographic that things may not sell, but no one can possibly believe any system doesn't have a large spread).