For me the answer is multi-fold.  For one thing, ground clearance!  But I think we've grown accustomed to long gear legs for the electrics that are swinging 21" props.    The main reason is speed/torque.  The motors (YS) produce a TON of usable torque.  You can load them up and they'll just keep pulling.  So when you say 19x6 vs 17x10... I was running a 19x10!  So i'd need a 22" prop to get down to a low pitch.  

Lastly and maybe most important, is noise.  Prop noise is a major contributor to the noise output, and with strict noise limits, prop size is a critical factor in making the noise requirements.  Too big a prop at too high an RPM will put you over the Db limit in a hurry. So the mantra is high load, lower RPM, lower diameter.