It's not actually a technical limitation, dual link DVI can support 1920x1200 @120Hz, though that's right on the edge of what it can handle. (The next common resolution up, 2560x1440, is out of it's range at 120Hz.)
I would guess that there aren't any 1920x1200 @120Hz monitors for the same reason there are hardly any 1920x1200 monitors, 1920x1080 is cheaper to make, and the masses buy whatever is mass produced.
As for display port, it's not all that rare on modern monitors, but compatibility isn't the issue, it's that companies don't like 16:10. (Or high resolution, for that matter. Is it really so much to ask for a single 2560x1600 24 inch screen when we've seen higher DPI in laptop panels for years?). If companies keep producing the same standard for years and people keep buying it, they aren't going to start doing anything new.