Climalife: why so many refrigerants?

To paraphrase the famous song – How many refrigerants must a technician install? - and the answer my friend, isn’t blowing in the wind. How many times have you heard it said, “there are just too many refrigerants, it was better when there were just three!”. Well perhaps I’m showing my age a bit with the second part of that comment but believe it or not there used to be just three mainstream refrigerants used for the majority of applications, so how did we end up with the plethora of refrigerants available today and how can you decide which refrigerants you should be using for what application?