Probably because the Desire was released less than a month ago and the iPhone 3GS was release almost a year ago? And also, once something becomes popular it is hard for competitors to persuade customers to purchase their product even if it is better. Even if Mac OSX is a lot better than Windows, for example (and I'm not saying it is), a lot of people won't even consider it because Windows is by far the most popular.
The web experience is OBVIOUSLY better than it is on the iPhone:
- Supports Flash Lite
- You view it on a 3.7" OLED screen
- Higher resolution than the iPhone, so it's better for browsing - can see more of the page at once
- The Snapdragon processor renders pages faster than the 3GS
- Bookmarks widget on your home screen
- You can download and save files
And the browser uses Webkit, so it should render pages the same as the iPhone