"I've never understood why people bash it so much, since I've stayed in some really unsatisfactory places, and Fishing Bridge was not bad at all.... I guess they expect Yellowstone to have very large sites with thick foliage and lots of space in between for privacy like some state parks. It's not like that, but to me it was perfect for busy sightseers. "
Federal campsites like that are in the minority. The ones that come to mind are Lassen, Bryce and the North Rim of the Grand Canyon. Even the Yosemite CG are tight and if it wasn't for the overwhelming views people would complain about those. Almost all of these CG were built in the 1950's or earlier and were designed with much smaller RV's in mind.
The NPS refuses to upgrade them since they are hostile to RV campers IMO. The added revenue who pay for CG improvement in a few years.