Why wont R-22 work in a R-12 ac system?

The simple answer is that an R12 system wasn't designed for R22. There are different pressures reached, there are different compressor oils used, and R22 may not even be compatible with a compressor designed for R12. Changing an R12 system to R22 could be done, but it would require extensive conversion, and would still be undesirable - R22 is being phased out, largely because it's a carcinogen and because of the harsh environmental effects.