I think it just didn't sell well in the USA [?]
Most likely the marketing dept of SCEA find it too hard a concept to promote any game in the USA that doesn't contain a gun toting Marine, gang member, alien etc, insert typical FPS.
The writing was on the wall when the added content for PULSE wasn't, and still hasn't been released in the USA.
Wipeout is the best game, but my god, it's had some absolutely terrible marketing decisions over the years that have really hurt it.
Usually because it has traditionally been the "Showcase Game" for any of Sony's new technology.
I suppose we should be thankful that there wasn't another Wipeout made especially for the PSP GO!.
That would of killed any hope of ever seeing another Wipeout again judging by what a huge flop the GO! has been.