Why are musical triangles triangular?

It's easier to bend a triangle out of a strait rod.

The sound would probably be a bit different but not different enough
to want to go to the trouble of bending a circle.

 

I think Antiphon is right about the actual reason why it's a triangle, out of convenience, but also it's a shorter distance between the three sides of a triangle than a square or a circle, using the same amount of material, so you can strike it faster.

but the sound would be a different tone for a circle than it would a triangle. I think the sharp angles in triangles effectively make it three different lengths of metal as far as fundamental harmonics is concerned (depending on the material and how it creased).

A circular triangle would be like a longer string, effectively emitting one deep tone instead of three higher tones.