I've always personally compared their level of fame with how we as humans hold up famous people. For instance, the princesses are analogous to politicians in our world. Celestia and Luna are the Equestrian version of the President of the U.S., they both not only rule their own country but make a huge impact across their own world so are renowned and respected far and wide. Cadence is more like a governor, well known in a certain region, but less respected and known elsewhere. Twilight is the newcomer and is more like a Senator, fairly well known, but hasn't contributed enough to hold a lot of respect and fame. As for the Elements, they are like war heroes in our world, they get their attention and fame for the day, but quickly fall back into normal life soon afterward.

maybe she can't because she has never been there before, I BET YOU NEVER THOUGHT OF THAT M8