Why does an Uno have ports B, C and D, but not A? At first I thought it might be to prevent any confusion with the analog inputs (A0, A1, etc.), but now I know the Mega does have a Port A! So, if that's not the reason, I wonder what is... ?
Most likely because the MCU used as the basis for the original Arduino Uno, the ATmega168 and later ATmega328, does not have a port A.
It looks like the answer is that the earlier 40-pin DIP ATmega16 had four full 8-pin I/O ports: A, B, C, and D. Port A shared its 8 pins with the A/D converter (A0 through A7). In the ATmega168/328 they went to a 28-pin DIP and didn't have enough pins for four ports. They took out Port A and moved 6 of the 8 analog input pins to share 6 of the Port C pins instead.
I get it, thank you!