The only problem with that circuit is that it needs an external power supply. It could be derived from the high voltage present on the flash trigger contacts.
However, look how simple it would be to incorporate it into the camera making it compatible with practically any flash ever made. Actually, with the right choice of opto isolator, R2 and SCR1 would not be needed making it a two component solution.
There can only be two reasons for not incorporating it:
1. Cost cutting (but not much).
2. Marketing. Forcing camera buyers to buy the matching compatible flash units.
This circuit could equally be fitted into older flash equipment making it safe on any camera.