If it was really down to two engineers, it's almost certainly what one or both of them were already comfortable or familiar with and no other reason. Six months is such a short time frame for long term projects like this that I imagine they could not spare much time for analysis of alternatives.
If you're comfortable with nextjs, you should be even more comfortable with a nodejs SSR application. It's the same thing, but simpler. The HTML doesn't even have to be pretty. We're really just querying a DB, showing forms, and saving forms. Hell, use PHP if you want.
