The designers figured that youd use X and Y for looping, indexing etc, and use A for adding and subtracting, shifts etc. So they saw a need for INX and INY, but didnt see a enough of a need for an instruction to increment or decrement the accumulator. @swiss vintage computing
65C02
@MauryMarkowitz There are a lot of instructions that could be added to the 6502 to decrease code size and/or increase performance but weren't. The designers were trying to make a cheap microprocessor, not a fully featured one and they drew their line with