A lot of it also has to do with what was available to watch on TV for the older crowd that grew up in the 50s-60s. For example, in the small town I'm from in NC, according to my father and mother, they showed the following teams all the time on TV: Lakers, Celtics, Sixers, Yankees, SF Giants, Dodgers, Rams, Redskins, Cowboys, Steelers. And I'm sure some others I've left out.
Point being, if you were born and raised in a state without professional sports and didn't have family from an area that did, you picked a team on TV/Radio and started pulling for them. My dad's family is from VA and his uncle had season tickets to RFK, so we are Redskins fans. In baseball, he is a Giants fan, my mom is a Yankees fan and I'm a Braves fan (because all of my friends pulled for the Braves as they were the most local team and always televised on TBS).
My dad was a Celtics fan and my mom cared less about pro-basketball. There are a lot of factors involved.