[huthuthike]

Biplanes have twice the number of wingtip vortices, so lots more drag. I suspect the reason biplanes existed in the first place was due to materials constraints. We didn't know how to make the wings strong and light enough to be very long, so kept them short and added a second set. This caused both more lift and drag. The drag was compensated for by running the engine harder (or using more powerful ones).

We now have sufficient materials knowledge that we don't have a problem making wings long enough to lift the plane. We're instead working on reducing the drag, so we can make engines work less hard.