There are a couple of different issues here. One is the history of western music in general, the other is the development of popular music traditions. In the first count, Spain, like England, was somewhat isolated due to its geographic location, thus, musical traditions there developed without as much interbreeding as the French, German, and Italian music traditions. Also, Spain's proximity to Northern Africa led to more interaction with what we think of as middle-eastern influence (a lot of the modal differences could be attributed to that). Some of the modal qualities that we associate with Spanish guitar playing are part of the idiom of that particular instrument, some have to do with the different influences on Spanish music that were absent in other Western European countries.