Why GRB format for programmable LEDs?
The physical arrangement of the LED dice is in that order (photo from Pololu), which would represent the physical arrangement of the shift register on the chip.

As to why you'd want to put the red die in the center of the lens, I would speculate that it is because red is typically the weakest of the three LEDs in terms of light output per mA so placing it near the center gives it a better chance.
Note: this is just speculation, but it holds together. Note also that the chip version of the WS2811 is shown with RGB data arranged in that order, so the chip designers had the notion to connect them to LEDs in that order.
The LED manufacturers (typically distinct firms) decided to scramble the outputs for some reason of their own.