Good question！In Japan, there are actually four patterns to describe prefecture; 都、道、府、県. The number of prefecture is 47; 1都（と）、1道（どう）、2府（ふ）、43県（けん）, which is quite similar to ”States” in US. The difference is historical rank. Long time ago, the central area for government was called 都(みやこ or と). As you might know, Tokyo is central area for politics since Edo-era, so 都（と） can only be used for Tokyo, so Tokyo ken or Tokyo fu is wrong. Next one is 府（ふ）that means ex-central area. Before Tokyo, Kyoto and Osaka area have been the central area for politics. That's why 府 can be used for only Kyoto and Osaka. Next one is 道（Dou）, it can be only used for 北海道(ほっかいどう). Historically, 北海道 was not Japan but the place under other country rules and Ainu people living as aborigine. So, it's a bit different from other area. Rest of other prefectures are kind of new prefecrtures; it's too many to distiguish eath other, so these are called 県, so most of prefecture could be categolized 県. Hope I can answer your questions!😄

Actually, 府and県 are same functions. But Osaka and Kyoto are traditional prefectures. So I suppose, only Osaka and Kyoto use 府in stead of 県.