IMO: NFPA sets the standards. With the cost of fire trucks and the hotter/bigger, faster burning fires we face today, they need to get as much water flow (GPM) as possible so they have gone away from the low gpm bumper mounted units.
Back in the day our trucks had to do everything but today there are special brush trucks and mini-pumpers that are low gpm units. All most all now use a midmounted pump to get 1500-2000 GPM. My old 1951 was a 500 gpm pump and it was top of the line equipment back in the 50's.
I hope that helps explain it...
