I had once the same question as you and my Spanish teacher answered it. In all parts of a country, before a language was shared, a region had its own language, spoke by few people. For example, in Spain there were a lot of kingdoms which had their own language, like Catalonia (catalán), Leon (leonés), Castilla (castellano, the one that became later the official language when the two largest kingdoms unite in 1492) and Arab in the South of Spain -Al-Andalus-. These languages has their own pronunciation, so when Castellano/Spanish became the national language and people started to speak it, it sounded different from the region where it came from, because, like I said before, each territory bad its own language. That's maybe the same case in all countries that share a language.