Diameter
You don't need to spend a lot of money or be very technical in order to have your own laser tube cutting. Anybody can cut like no one has ever seen, from students to engineers.
Imagine having to place a through-hole centered on a tube. It has to be centered to the actual dimension, not just one face of the tube. If the tube is bowed, that’s going to make things more difficult. That’s the life of tube fabricating.