Many? Most? satellites going up these days are in close-to-polar orbits, so you’d like the rocket to launch near northward or near southward. So heading south out of Vandenburg is fine, but not out of Florida. Is it for practical purposes impossible to launch the rocket NEward or SEward but somehow get satellites into 80-deg inclination orbits? How about from Brownsville?
Yes you can do technically do it. The problem is that you don’t want spent stages or debris from a failure to land in populated areas. Going either north or south from Florida has this problem while going south from Vandenberg doesn’t [it is flying over the ocean]. You can do a dogleg maneuver from Florida where you fly east and then turn and fly north or south–but this consumes extra fuel.
Rare but doable. Space-X did it in 2020, which I believe was the first since 1969.
Energetically, it’s about equally easy to launch into a polar orbit from any launch site anywhere on Earth. So other considerations take over. On the one hand, there’s a lot of launch infrastructure in Florida, but on the other hand, you don’t want debris fields right along the coast. And some polar launches are for military or other sensitive payloads, so you might want to launch them far from where any civilians can see them. And there’s a lot of infrastructure at Vandenberg, too.
What’s not easy is to launch into an inclination lower than your latitude (theoretically possible, but it takes a LOT of fuel). So, for instance, the International Space Station needs to be able to rendezvous with launches from both Florida and northern Russia, and so they put it at the Russian inclination, because Russian launches couldn’t reach a Florida inclination.