Not sure if this reason was mentioned...
A big reason is that it is important that people can play their existing standard DVD selection well and, the perception is that, for the money, good quality standard DVD players will upscale standard DVDs better than any HD player in the same price bracket.
That's why some people still pay £1500+ for a "standard" DVD rather than a similarly priced HD or Blu Ray player.
Anyway, that was my reason (or excuse) for going for a (much cheaper) Denon 1940. I was very much planning on staying with standard DVD... until I saw the highly rated (5 stars in WHFIS&V) Toshiba HD-EP35 HD-DVD Player selling for £269 and coming with 8 free HD DVD's and a free HDMI cable.
Sure it wouldn't upscale as well as those elite £1500+ standard DVD players, but neither will the Denon 1940. Perhaps the Denon might upscale a bit better but probably not that much more. Although it wasn't my original intention, as a "bonus", it can also play HD DVD's. And it's cheaper overall. So HD DVD here I come.