Why does a tuba make a low pitch sound?

Pitch is determined by the frequency of a sound wave. On a guitar or piano, the wave is established on a string, but in a wind instrument like a Trumpet, Flute or tuba, the wave is established in the column of air inside the horn. The longer the column of air, the slower the wave will be, and slower waves produce lower pitches. It's just like on a guitar, where the longer portion of a string that's allowed to vibrate, the slower the wave and lower the pitch will be. All that piping in a tuba encloses a very long column of air, so its pitch is low, while the short pipe of a flute produces a high pitch.