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.
Porting a PSP game to the Wii is very easy, it would take more effort to make a port bad than to make it work on a more powerful system.
There is no logic in terms of audience; the Wii sells software, the PSP does not. Split/Second was developed to be released in 2010, at a time it was already known that even big series like Need for Speed can only get about half a million in sales on the PSP, meaning sales in the same ballpark as what NfS achieves on the Wii.