BillO wrote:
... did WDC decide to replicate the 6522 rather than the 6526? Or even take the next step and produce both and drop the 65C21?
Seems to me the 6526 would be a more generally useful device.
I suspect WDC didn't do the 6526 because that was a Commodore Semiconductor Group (CSG) scratch design from the era of Jack Tramiel. Bill Mensch would have needed a licensing agreement from Commodore to legally produce a clone. There has been some off-and-on conjecture that Mensch didn't leave CSG (formerly MOS Technology) on the best of terms, which would have made unauthorized cloning of the 6526 a hazardous undertaking, lawsuit-wise.
As the 6526 was only a slightly-enhanced version 6522, I doubt there was a good business case for it. Also, it too had hardware bugs, two of them actually, one of which was more serious than the serial shift register bug in the 6522.
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(Perhaps you could generalize the question, like why only the 6521, 6522 and 6551.)
Back when WDC went into business, those three devices were the most widely used with 6502 systems. There was a market for them at the time. Nowadays, I suspect the overwhelming majority of uses for them is in hobby systems.