Cliff, great question! I had never thought about it. Does it have something to do with ancient languages, like Latin? But I'm not sure what "Florida" or "Libya" have to do with Latin.

How do you know if a significant number of countries or states end in "a"? Is it more than 30%?

Well, this IMO is an issue with english. In german, Austria is spelled Osterreich (for example). In german, it is very common to call a country a land/empire (reich) of "nationality": Deutchland (Germany), Frankreich (France), Russland (Russia)...