Because non-releasable bindings are just plain dangerous no matter what length of skiboard. My son just completely fractured his tibia due to non-releasable skiboard bindings. I've been riding the non-releasables for 2 seasons now and I have to admit - I've been close to screwing up my own legs a few times too. I really thought that the skiboards were short enough to get away with non-releasable bindings, but I learned my lesson the hard way.
Your best bet is the Spruce Mountain setups that still use the 4x4 inserts and minimize the effect of binding on the flex of the board. If your skiboards already have 8-hole type cheapo plastic bindings then you could also go with a full flexing type binding (like the Railflex that skiboards.com now sells).
I've retro-fitted my son's skiboards with Tyrolia SL 45 Railflex Lite bindings and I've purchased Spruce Pro Prime Risers for myself.