Perhaps because HD got railroaded by the TV people and now it's commonly taken to mean 1920x1080 rather than a DPI measurement?

Also, your "more technical" definition is off by a long shot; people have been using 100-125 DPI screens for years, that's 100+ and it's not what would pass for a retina display unless it was 40" across and you sat at TV viewing distances away. Retina is being used in the generic sense to say "2X the DPI you're used too", or 250+ DPI for the current examples available. But even that DPI measurement is flexible because it's related to viewing distance.

So rather than stretching for generic terms that dont actually make it any clearer what you're describing, lets just stick with "retina" until we really have a better term :)