Join Date: Oct 2015
Location: U.S. Rocky Mountains
It's all about the wheel diameter... 21" and 18" wheels have always been wire spoked, so the tires in these sizes have always been tube type.
And to this day, Dual Sport bikes continue to use 21" & 18" wire spoked wheels, for a number of reasons. At the top of the list is the significantly lighter unsprung weight over that of a cast aluminum wheel, as well as a much wider variety of D/S and knobby tire choices.
The simple and easy solution to the OP's question is to use a product called Slime (google it) in the tubes.