I guess you can offer a better explanation why a third party ports a HD game to the PSP, but not the Wii, in the year 2010. And remember, this is about a game that gets its vast majority of sales in regions outside of Japan.
I would imagine it's purely technological - I'm no techspert but I'm guessing the cost of porting / developing Split Second on the PSP was alot lower then if they were going to make a Wii version that was actually good. Maybe they didn't have the resources to make a Wii version of the game which wouldn't end up like the Hot Pursuit port did.
I'd imagine there is some logic in terms of audience too - the Need for Speeds and racing games in a similiar style have never set the Wii sales charts on fire, whereas the PSP has 3 NFS million sellers as well as Gran Turismo, Midnight Club, Burnout.