Because you only have a certain amount of alternator power. At 12V say it is a 500 watt heater then you are pulling 40 amps from the alternator which, on a cold morning will also have to power the lights, a fan, rear window heat, heated seats, all the electronics AND recharge the battery ready for your next start.
I never understood why cars did not move to a 24v system like small aircraft. I guess it was just the awesome weight of the Big 3's inertia. M-B tried to go to a 40 v system in the '90s IIRC but gave up. More volts less amps, thinner wire, less cost of copper, lighter cars.