The 68xx range wasn't viable for low cost machines until it was too late and Commodore was already deeply invested (as in, it owned MOS).
Keep in mind that the entire reason for being for the 6502 was that Chuck Peddle had tried (and repeatedly failed) to get Motorola to accept the idea of pushing a low cost design. At the time the 6501/6502 was introduced MOS charged $20 per CPU while Motorola was charging $175 for 6801's. Motorola dropped their price to $69 near instantly, but that was still a substantial percentage of the cost for a typical low end machine.
On a machine with more memory it might have mattered, but for a machine constrained to 64KB, the 6502 instruction set was easy enough to work with - it's not like you could afford to use a large stack most of the time anyway, for example.
Keep in mind that the entire reason for being for the 6502 was that Chuck Peddle had tried (and repeatedly failed) to get Motorola to accept the idea of pushing a low cost design. At the time the 6501/6502 was introduced MOS charged $20 per CPU while Motorola was charging $175 for 6801's. Motorola dropped their price to $69 near instantly, but that was still a substantial percentage of the cost for a typical low end machine.
On a machine with more memory it might have mattered, but for a machine constrained to 64KB, the 6502 instruction set was easy enough to work with - it's not like you could afford to use a large stack most of the time anyway, for example.