Sanctioning rules a side. There are 2 reasons companies put larger than .21 in buggy kits in my opinion.

1.) Bigger sells if someone off the street walked into a hobby shop and saw to RTR buggies for about the same price one with a .21 and one with .28 they would most likely buy the one with the bigger motor.

2.) Entry level .28 are cheaper to make and will have better performance than a cheap entry level .21. It cost almost nothing in material and tooling to add .07cm to a sleeve and piston to fit in a .21 block. It cost a lot more money to tool a production run to manufacture a performance level .21 ( percision porting , timing, crank shaft, etc)