You should also factor in that the last wipeout release that was playable on a TV set was almost 10 years ago,so you have a whole heap of players that basically only know the PSP versions and because of no playable demo being available don't realize how huge a change it is.
I think you also have to factor in that some people just see HD as a remake of Pulse,so they haven't bothered.
Plus they might think they are going to get stiffed in the added content dept [Pulse in N.America]
I don't think sales in Japan have been very good if the Japanese online presence is a way to gauge sales there.

But I think the biggest blow to possible sales was releasing HD as it was with no campaign difficulty level adjustment prior to the patch.
People just found it too hard,and that fact got spread by word of mouth.
I remember the download night,must of been 30 or so people at the Australian PS forum all getting it at the same time,out of those I've seen 2 playing WOHD in the last 2 months,most gave up after 3 weeks.
I wonder how many people have WOHD installed and have given up prior to the patch because it was too hard and don't realize that the patch exists.
I don't remember seeing anything [news] reporting the patch officially on any Sony site,and a lot of people do not visit gaming forums,you just have to see the amount of new posters at the HOME site to see that.

Hopefully some DLC will spice things up a bit,plus the Xmas WOHD promo/temp price drop and playable demo.