Eric-Jon Waugh: For about four years, no one significantly built on Pong. You saw things like four-player Pong, and Pong with two paddles, and a vertically-oriented Pong that passed itself off as a Volleyball sim.
Atari did experiment a bit with Gran Trak 10 and Tank!, but somehow it took until 1976 for Bushnell and Bristow to hit on a one-player version of Pong. And that pretty much was the missing piece that gave us two distinct schools of design, the home PC, and thereby the information revolution that allows a person to research articles such as this.
As these things go, Breakout was pretty well-named.