I know how the tubes work, but remember, one company, one single company, builds essentially ALL of the SPG's for the global scuba market. That's a LOT of gauges. They have to produce 1", 1.5", 2", 2.5", 3" gauges so that is 5 internal parts on it's own. If they had multiple scales, that would essentially double the number of parts and inventory that they have to maintain in order to keep supply. This increases manufacturing costs because of logistics, not because the actual product parts are more expensive.
Typical round speedo in a vehicle with 180* sweet. 120/140/160mph is always easier to read than 90 because it puts the "highway" speeds of 50-70 at the top middle of the gauge, right where you are eye is going to have the easiest time looking at it. A 90mph gauge would put your normal highway speeds at the very end of the gauge and that makes reading more difficult. That is why you rarely will see speedo's with a smaller swing.
The current 5000psi gauges are basically the same. The "normal" pressure ranges of 0-3600psi are in the top 180* swing of the SPG. Very easy to read. You want something easier to read with better precision? Go to a larger face gauge.