In addition to what those above me have said, Fluorite has been popular since the opening for a number of reasons. Like mentioned, it IS the biggest map--it has the most objects, more space, and you can seclude yourself rather easily if you want to get away.
The only original maps in FH were bonfire, ficho, FP, the cape, and temple of dreams. If you think about those maps, you can see why FP got so popular. You originally spawned in Bonfire--and it first, it was the most popular map actually. But due to the lack of space, the chat would move really fast so people would leave. The tunnels, though cool, are a bit rather boring and once you go over that waterfall there used to be no way back unless you did "Go Home" or the screenshot jumping glitch which only worked some of the time. So once you got to FP, you were sorta stuck there. The cape is merely for custom worlds, and temple is just...probably the least popular public map. It was cool at first, but once you got to the top, there was nothing there, so you fell back down.
So really I think at the core, it was the lack of easiness to get out of FP, the space, and the good advertising pool.