Sinclair was a company particularly sensitive to BOM¹ costs. They were
willing to put in quite a lot of development effort to have a machine that
was a cheap as possible to manufacture, and selling computers at low cost
was their main competitive advantage.
Commodore, on the other hand, was not terribly sensitive to BOM costs.
(Compare, for example, the Apple 5.25" diskette system to Commodore's
drives; Commodore drives had a lot more parts, including a CPU, I/O chip,
RAM, and ROM where Apple got by with a handful of small-scale integration
7400-series parts and a ROM or two.) And Commodore didn't have to be quite
as sensitive since they owned a chip manufacturer (MOS).
Where Commodore was quite sensitive was development cost and speed: read
up on the history of almost any Commodore computer product and you'll see
that Tramiel was always pushing hard to get a product developed and out for
demos at the next trade show.
For the VIC-20, using a couple of 6522 VIA chips offered several advantages
in this regard:
- Using a more sophisticated chip driven by software makes for easier,
faster and cheaper development than building the same functionality out
of more basic electronic building blocks.
- In the case of the VIC-20, they already had a computer (the PET) that
used the same chip² and had software written for it. Thus they could
re-use much of the existing hardware design work and software (e.g., for
the keyboard, cassette tape and "user port" expansion interfaces) rather
than having to develop the hardware and software from scratch.
- Since Commodore owned the company that produced the chips, they
effectively bought them at cost, rather than at cost plus a substantial
markup to allow the chip vendor to pay off development costs and make a
Essentially, being able to reuse much of the PET design saved a lot of
development time and got the computer done sooner, and the additional BOM
cost didn't hurt Commodore as much as it would have hurt another company
that did not manufacture the chips in house.
¹ "BOM" refers to the "bill of materials," which is the cost of all
the components used to build one unit of the product.
² Actually, the VIC-20 used just one 6522 VIA and two 6520 PIAs.
However, the VIA was basically just a PIA with a few more features added.
Especially if you're the manufacturer of them, they probably weren't
significantly more expensive.