Because prolog/datalog don't offer a list of questions that you can ask based on context to calculate someone's US taxes.

That's the database you consult(). Doing income taxes is well-suited to traditional logic programming.

This is a bit like asking "why would I use my car's schematics instead of a wrench".

This is the rules engine's details. You could use it to build the logic and traversal in whatever language you like.

I think they're asking why you would build a rules engine and fact graph instead of "just" encoding it in Datalog.