Because it is a BUGLE not a TUBA. It has a more conical shape throughout it's tubing than a Tuba does.
Go to the manufacturers websites and see what they call them. You will find that they ARE in different families. Concert horns can have different pitches (i.e. Bb, C, F, E) but only BUGLES are pitched in G. There are no concert horns pitched in G that I am aware of and no Bugles in any key other than G.