Why does MakerBot Replicator heat the extruder and platform separately?
It really doesn't quite have enough power to heat everything at once. Initially bringing the bed up to temp takes a lot of current and so Makerbot's start sequences decrease stepper current and hold off on heating the extruder(s) until the bed is preheated. Once preheated, the bed's power draw decreases to a lower "holding" level and there is available capacity to do everything else.

The start sequencing decreases the max average power draw by a pretty substantial amount. The decrease is about 0.6A each for the XYAB steppers and 1.7A each for two hot ends, totalling 5-6A of avoided peak power draw. (Even lower if you preheat with the steppers disabled rather than idling.)

The history here is that one of Makerbot's design decisions with the Replicator 2 and 2x was to utilize off-the-shelf UL-listed power bricks (like a laptop might use) instead of an internal power supply. This made engineering and safety certification much easier, and completely eliminated mains-voltage wiring inside the printer, which is a big safety plus. However, the largest readily-available 24v power brick had slightly less current capacity than optimal. So Makerbot came up with a software workaround for the hardware limitation.

If you want, you can replace the stock power brick with a ~350w PSU and preheat everything at once. Sailfish firmware builds intended for printers with properly-sized PSUs (eg FlashForge Creators) will allow simultaneous preheating.