Cost. A good TF-2 kit is around $80 for the kit, and takes at least an hour or more to install, so the customer would be looking at close to $200+ to just install a shift kit. A manual VB is not much more expensive and operates at high pressure.
It really depends on the application and the customer. A guy building a trans for racing only usually wants the reverse manual VB, and the guy looking for a low budget rebuild won't want to spend the $$$ to have the valve body built.
It usually takes a customer who has a hot street car and is willing to spend the extra money to have the transmission built right.
You also want to find a shop that knows performance modifications for Mopar transmissions. Many shops hire guys that just do general rebuilds with no modifications, so they may not know all the tricks and setup of a modified transmission.