People have strong ties to their origins and regions of birth. It's not something that people easily give up. Regional loyalty is everywhere and in every nation. If you look at every country in Europe (as an example), you'll find groups and subgroups of people that remain loyal to their ethnic origins.
Here in the US it's the same. Americans that are born in the West and South don't relate to the people in New England. Their foods, local customs and manner of speaking is different. Southerners are very proud of being "Southern". Im a native Texan, now you wanna talk about pride! Texans are very proud of their state, their history and the fact that we were the only state in the US be have been an independent republic before joining the Union. There's a couple of very popular sayings among Texans..."Don't mess with Texas", & "American by birth, Texan by the grace of God!"
We're all Americans, but each State is it's own region or part of a region and that's what makes a country great! Why would you want a country that's the same in every region? If you would visit a country where everyone is the same, looks the same, eats the same foods, dances to the same music, dresses alike with no variation, how interesting of a culture would that be?
That's why Spain has a rich heritage...because Castellano is not the only language spoken there, because flamenco is not the only form of dance and because paella is not the only dish that's eaten.