It's still good to see that we still rank above all the cities in Florida though. We still reign supreme in that aspect!
Well, as others have mentioned, it's specifically because of the city-county consolidation. If Miami consolidated with Miami-Dade County, they would have 2.7 million people living in the "city." New York's numbers are geared the same way (although they are much more populated) in that up there, what we would call a "county" they call a borough of New York City, and thus, five "counties" are making up one city of population.
Besides, numbers aren't everything anyway. The "status" of being the most populated city in Florida hasn't exactly set Jacksonville on fire, eh? And considering the state of Floridian development, one could make a strong argument for slower growth being better than rampant, out of control, "Orlando/Miami/Fort Lauderdale" growth, especially if there's no plan or politics supporting intelligent urban planning.
This focus on the number one ranking entirely misses the point. Yeah, Jacksonville "beat" Miami on a technicality, but if that ranking is all one can hold up...who cares? Focus on making Jacksonville the best place it can be, and stop paying attention to the numbers. Because they'll come when you focus on making Jacksonville the best place it can be.
Why isnt the same thing happening in the other major floridian cities then?
Because they took part in the urban rediscovery of the last 20 years or so, especially in South Florida and Metro Orlando. Jacksonville sat that movement out, for the most part.