Why does the '51 not have a MAC instruction?, why is Iram only 256 bytes, why does a Toyota have 128 horsepower?

Because the designers decided so.

When anyone design anything there is a weighing of cost and fit vs benefit. Anyone that has been through that process know that whatever you do, someone will bitch about it. The 'game' is to keep the number of bitchers as low as possible, zero is impossible. The designers of the '51 architecture balanced wants (4711 diffent instructions) against possiblities and cost and came up with 255 opcodes they judged to be the best set for cost and performance.

Erik