Intermediate kits in the 2.5-3 gallon range don't have a very good fit to make them more widely available. You can brew two or three gallon batches on a five gallon kit. Three gallon cornies are out there but not always easy to find and often cost the same as five gallon kegs. That may trap people into bottling, which many do not like. If you decide you want to move up to five gallons you have to make some additional purchases but moving down from five to three does not require spending more money.
That said, there's no right kit for everybody and it's good there are options. As recently as 2009 when I started brewing you bought a five gallon kit and that was your one option. Maybe a couple years after that smaller sizes started to appear as urban homebrewing became more popular.