South Florida is endless sprawl - except for the urban cores and some sections of the waterfront, it's pretty much all 5-houses-per-acre single-family homes. That's not enough density
to support anything other than local bus service at mediocre frequency, which in turn is not sufficient for any kind of meaningful mode shift. So you have a couple higher-density corridors like Metrorail and the coastal rail routes, but the vast majority of the metropolitan area is stuck with congested roads but little transit. It's the same issue that a lot of western cities (LA, Denver, etc) face.
Boston's density is concentrated - within a 5-10 mile radius of the core, you have densities of 10-15 units per acre in most areas (and higher in urban squares and villages), which is enough to support rail transit. That's a function of topography and centuries of village growth constraining where development happens. At full fantasy build-out, you're looking at a dozen 5-10 mile HRT/LRT/EMU radial lines plus at least one circumferential route, and a dozen 20-60 mile regional rail lines, all of which would have higher productivity than Miami. That's balanced by some very-low-density suburbs that don't need transit (and don't account for a majority of the population the way Miami's sprawl does), and a lot of parkland. When you subtract the huge swaths of forest that don't need transit, the remaining areas average much higher density than South Florida.