Simply because when you take into account that they have only been making tweaks to the handling and changed the interface, it makes it difficult to say it's a "totally new build", especially when they've made it pretty clear that all the existing cars from GT5 will be retained with a few models confirmed as being rebuilt as premiums, but not mentioned that this is the case for everything. All the tracks will be retained too, so there's a whole world of copy-pasting going on. All the "new" items could have been added as patches or DLC, yet instead it's being rebadged as a new game.
That said, until further details do come forward there's still only vague details that came out of the full presentation, which I watched yesterday. There's mention of "custom" parts, but no really solid details as to exactly what this will entail, bunches of wheels were shown, of which most were actually repeated across their preview screen, so there were actually only about 13 different wheels in total (same with wings). Will be interesting to see what it actually looks like in comparison to GT5 when the first demo comes along in July.