Why are racks preferred to tanks?
Tanks don't have good ventilation and for that fact they are not very good for keeping rats in.
Rats have a weak respiratory system, asf's and mice do fine in glass tanks, rats not so much.
I used tanks for preggo moms when I first started breeding, wire cages for groups and tanks for moms, what a chore...
And constantly having females getting red runny noses

Racks are just easier and not that much more expensive than setting up 10 gallon tanks and buying wire cages and shelving and....
IMO racks are well worth the small cost to build even for a small colony.