Because not alot of people keep them and when they do they usually are killed by tankmates or die for unkown reasons. They are a very fragile fish that we as aquarist do not fully understand......yet
There is a Aquarium here in the US that has a few in a massive display that are 14-15".
I have 3 scombs and have very good luck with them! They grow about an inch -2 inches a month for me and have proved tough against the narliest tankmates! Im hoping to pump a few 15-20" ones out!