The reason most dont come with it is cost. The big ring diff uses 12 balls instead of only 6. So it not only spreads the load out wider it sreads it over twice as many bearings. They are super smooth compared to the regular diff. It seemed to me when I switched over I could adjust the diff a little looser also. Thats probably why it feels so smooth. I know the CEFX car comes with big ring diff. Others may also.

BTW someone was just trying to sell one on the for sale forum for only like 35 bucks new.