Why did biplanes go out of style?

Question

Are they not efficient enough?

Answer 1

they are not cost effective.
there is too much drag,
there is too much cost to more wing surface that is not needed due to more powerful engines.

Answer 2

The big disadvantage of the biplane layout was that the two wings interfered with one another aerodynamically, each reducing the lift produced by the other. This meant that for a given wing area the biplane produced more drag and less lift than a monoplane. Once thicker wing sections and improved structural materials were introduced, removing the need for external bracing, monoplanes quickly superseded biplanes in aviation. Modern biplane designs now exist only in specialist niche roles and markets such as aerobatics and agricultural aircraft.

Answer 3

You are on the right track. Monoplanes have less drag and more powerful engines so they can fly faster. Biplanes were usually cloth over wooden frames. Monoplanes were usually metal and could stand up to the extra stress that the faster speeds produced.

Answer 4

Two wings were required in order to have enough surface area to produce sufficient lift given the weight of the planes and especially the engines. Later refining of engine/body design no longer required them.