Well, because Skyrim (same as Oblivion and Daggerfall) takes place in land of men, so it is only obvious that it resemble real world
TES3 was first game that took place in land dominated by mer (dunmer, to be more precise), so it looked different
Summerset Isle, Valenwood, Elsweyr, Black Marsh and Morrowind are those provinces that should be totally different from Earth
(Personally I really hope TES6 will take place in Elsweyr or Black Marsh)
While Cyrodiil, High Rock, Hammerfell and Skyrim should look like Earth
P.S. To be honest combining two provinces in one game, each of them different, is best choice. Few possible combinations for next games: Morrowind+Skyrim, Hammerfell+Summerset Isle, Black Marsh+Morrowind (explosive combination
), Elsweyr+Black Marsh, Valenwood+Elsweyr, Summerset Isle+Valenwood. Such approach could make game world more diverse.
P.P.S. I still expect to see in one of the further games all Nirn (or at least remake of Tamriel)