Not just one of two numbers. Ballots contain multiple referenda, and races for multiple positions, each of which can have anywhere between 1 and tens of candidates.

You need to display all of that information, take a user's input, store it in an auditable format. Oh, and people like "small government" so making custom hardware is completely out of the question. Using a "multipurpose" computer is the economical choice.

But there's another aspect to this: if you can swap out the hardware and keep the software the same, it makes the whole thing more transparent. A jumble of 4000-series CMOS on a breadboard could hide any number of bugs/backdoors. And, perception of trust is important, nobody wants to vote on your science fair project. Also, you'd need to produce thousands of these machines to run an election. Really easy to procure thousands of commodity multipurpose machines... but you're talking about mass-manufacturing your science fair project. Hell no.