One advantage that C has over most other languages is that it has a very simple and widely-supported ABI that can be embedded into virtually anything else under the sun, without additional dependencies.

That's especially valuable in this case because the kinds of organizations that have lots of business logic in Excel spreadsheets are unlikely to want to switch programming languages to take advantage of a tool like this. Given that automatically-generated code is likely to be ugly no matter what language it's written in, it makes sense to target the lowest common denominator that everyone can benefit from.