I would assume it is because we are in Skyrim, the native provice of the Nords. The meaning behind being an Imperial Citizen has changed since the time of Uriel Septim. Provinces have already left the Empire, thus, the prestige of being an Imperial has dropped.
I think a more logical way to reflect this is disposition bonuses instead of increased speech skill. Nords don't become better speakers the moment they cross the border into Skyrim. Imperials don't become worse speakers just because people don't like them that much anymore.
Disposition may not be shown with an indicator now in Skyrim, but it's still there, just behind-the-scenes.
EDIT: @dannythefool
So speech in Skyrim is now combined with something like Personality?