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.
Looking at the VGC figures I don't think it's hard to see the PSP had a better track record at selling Split Second type racers though. Clearly it has an audience, even if software sales have dried up it's still shown on numerous occations these realstic (In the loosest sense of the word) type racers can sell, whereas the Wii has a list of half a million sellers and a few slightltly above that which were released close to the consoles launch.
Neither really boasted amazing selling opportunities - but a handheld game is still the cheaper option which requires less resources and people working on it. I can't really get into a debate on the technology behind porting a video game, but from what I've gathered these PSP games which are on 360/PS3 (Dante's Inferno, Split Second, Army of Two) arn't literal ports of the PS3/360 but indepentely developed games that share alot of resources and that doing a similiar thing on the Wii (At an acceptable quality) would simply cost more money.
Fair enough if just Disney did it (PSP port) and it was a one off occation but we've seen it happen on numerous occations now from EA, Namco, Konami etc, so I'm assuming there's got to be some sort of technological logic in it.