There's just no other non-kite sport that is popular to the point where everyone thinks that being strapless on this type of board is the only way to go. In fact, it is quite the opposite, where this type of board (twin- tip) is more popular with straps than without, in non kite sports.
Since there is no popular emulation of the sport at the root, no one wants to put up with the handicap of being strapless on this type of board. And being strapless is a pretty big limitation to the entire kiteboarding system. Kite power winds up being limited, timing becomes critical to the point where only a single timing can be used, and changes midair are severely restricted. Overall you just wind up being able to do a great many things to a lesser degree, though a very few things can be added to the repertoire, when strapless.