I'd say it's mainly because of general ignorance for "lesser known parts of history in nowadays-popular-culture-determined perception".

For instance...Since the movie Braveheart came out, Scotland was featured in numerous computer games (MTW2 included) though it was really quite an insignificant kingdom. Yet people nowadas have a much better awareness of what may have gone on in Scotland, which didn't really change the course of European history that much, than they do of what went on in the Balkans or the Iberian penninsula, where eventually the spread of Islam was halted, thus resulting in a Christian Europe...

Kingdoms that may have been players back then, but are less significant today tend to be overlooked in favour of those that may have or may not have been important then, but are ore influential now. Thus a "Germany" or a "France" are featured in most medieval games, though they were as much a collection of warring little states as the Balkans were...