Having been involved in many technical discussions about alternative refrigerants, I will give you the best reason for only using R12 or R134a.
All other refrigerants are blends and can not be recycled. There are NO CERTIFIED tools to reclaim anything but R12 or R134a. if a car comes in with anything else in it it can not be worked on unless you plan to vent the charge.
That is the main reason but there are other good ones such as availability of refrigerant specific fittings (an absolute must for conversions in the US), availability of refrigerant for future repairs, and finally other than compatability with mineral oil none of the possible substitutes do any better (or convert any easier than R134a - just cause they say you can drop it in don't mean it will work, or last)
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician