Because no one would use it. The L and DZ are still pretty noisy with the long hatori pipes. In F3A competition you get bonuses for quiteness. Therefore why include something that will not be used. Remember these are competition engines. The 140 FZ sport, and all the other sport YSs do include silencers, but they are aimed at the average bod (like me) looking for something special.

It will slap you in the face and go into a corner to sulk.

You can use lower nitro on the sport YSs (I use 15% coolpower on my 120 fz) but 10% on an L or DZ and you are asking for trouble. They will run, but not reliably.