Because not everybody needs a wide-gamut monitor with plenty of room for calibration at what most users would consider low brightness levels? (Where "low brightness levels" means that monitor-white is not significantly brighter than "bright white" paper under ISO-spec lighting at 95 cd/m^2.) $1K-ish is cheap for a graphic arts monitor (compare with LaCie and Eizo), but it's a heck of a lot more than the average consumer needs to spend for their purposes.
Why do the new UltraHD monitors need to have the same name as previous Dell monitors? It's just a name. In fact, UltraSharp vs UltraHD seems like a great way to distinguish between 4K+ and lower res monitors, while still making both seem "good" to consumers.
Ultrasharp is to Dell monitors what "MacBook Pro" is to Apple laptops. Dell clearly intends, with their 24/32 vs cheaper 28, to have pro quality and mainstream quality displays both at UltraHD/4k resolution the same way they have both pro quality 1080p (ultrasharp) and cheap crappy 1080p (not ultrasharp).
It is unfortunate for dell that the 2160p resolution got labeled something so similar to their existing brand.
Sure. But as someone who has owned several Ultrasharp monitors, I don't feel that it would hurt their brand to come up with a different differentiator for 4K monitors. The brand is Dell, not Ultrasharp. I can almost guarantee that anyone who even knows what Ultrasharp means would be the kind of people to research the specs of a monitor before buying it anyway. So maintaining that 'brand' across all their high-end monitors just doesn't seem necessary.
Also, how about just a simplified product line like, I don't know:
Dell UltraHD 24" Dell UltraHD 32"
Along the lines of Cinema Display.