1) In cases of low power, Factorio transfers power proportionally to demand. If I give factory buildings huge power throughput, then they will receive most of the available power, making brownouts much more severe since the coal mining drills will be the first things to stop working.
2) A higher power transfer rate requires a larger energy buffer. If factory buildings would start out with a higher power transfer rate, and therefore a larger energy buffer, your early-game factory would shut down for several minutes whenever you place down a factory building, because all your steam engines would be busy pumping all their energy into that energy buffer. In fact, during that time you'd likely have to fuel your steam engines manually if your fuel source and power setup depends on any amount of electricity.
