It's been a while since I did anything relating to construction so forgive me if I'm getting something wrong. Bitumen is the byproduct, and it's cheap because

It's been a while since I did anything relating to construction so forgive me if I'm getting something wrong. Bitumen is the byproduct, and it's cheap because of that. But IIRC it wasn't just the cost that made it attractive. It's easier to work with then concrete, and maintenance via resurfacing is easier as well. It's a fair bit more flexible as vs concrete which can be desirable as the soil under the road settles and shifts over time; a concrete road will crack under those circumstances.