Pace make a very nice frame from rectangular section tubing and there was a cast magnesium frame (Kirk) with an I-section, but triangular sections may concentrate stress at the edges too much. Circular sections dissipate stress evenly.

A diamond bike frame is actually not ideal for resisting the torque or twisting force, but it has to be flat to permit a rider to stradle the frame. Most bikes are sufficiently strong to resist the bending, but longer tandems do have problems.

In addition to diamond frames, you get space-frame designs, cross-shaped frames, and monocoque frames.