Are Long Tanks Rare?

I am looking to upgrade the housing for my goldfish. I went to Pet World
this afternoon and was surprised to find that this pet supply warehouse
did not have any 30 gallon long or 40 gallon long tanks. I also stopped
at an aquatics store that had many tanks for sale, but not a single 30
gallon or 40 gallon long was available.

I currently have a 4-inch Ryukin and a 2-inch google-eye in a 20 gallon
long tank. I want to move them up to either a 30 gallon long or a 40
gallon long and add a third room-mate to the tank. I currently have 2
pearlscales in a 15 gallon, and I'd like to move them to the 20 gallon
long after the ryukin and google eye move to their new tank.

Does anybody have any ideas on why 30 and 40 gallon long tanks appear to
be hard to find?