Most likely due to all the extras needed for gas engines like spark plug, stator or alternator(not needed but better than pure battery) some sort or spark timing system and probably more. also in such small scale gas probably won't produce much power. If you take that 23 cc and make a nitto motor of the same size it would probally have more power but need much better cooling.
I'm not saying making a smaller gas engine isn't possible but it's just not practical.