I think it is a cost thing,, I used to design bikes for a well known company and we wanted to do a small number of aluminium frames in the uk
I went to Harris performance ( used to work there as well) who build very nice motorbikes in aluminium and steel , but the prices were way too much ,,
the main problem with aluminium is you either use 7000 series alumium which doesnt need heat treatment ( if i remeber rightly) or 6000 series which requires expensive heat treatment and jigs
the problem is 7000 has limited stock sizes
carbon requires v expensive tooling for each size , and an even more epensive autoclave ( big pressurized oven) to cure the carbon pre preg
( woven carbon material impregnated with the correct amount of resin)
you can of course use a wet layup carbon where you brush on the resin like doing glass fiber and cook it in a normal oven or even a fan heater but that makes for very heavy and reduced strength
so to some it up you have a jig a tig welder , brazing torch and some bits of tube and a hacksaw , then you can build a steel bike it's not rocket science
i may have not remembered stuff correctly but I am sure , no certain someone will put me right