There are a few killers...
Parts Availability - The downtime caused by no parts in compay is not good if your relying on that machine for your income
Price - Sure, you can buy one but the cost of even converting a machine is the high initial cost.
Training - Since the 172 is most common use is training, sure, you can learn in a single lever aircraft, but whats the point? When you go for a job on anything larger guess what, you need to know your engine management and how to handle a constant speed prop and mixture control. Until there are affordable conversions for your light twins that are used in charter (and that are at a suitable weight that they are not going to use what little useful load is available), time in the 172 isn't really going to help.
STC's for other aircraft - Cost of fitting them an aircraft is high enough, yet alone having to develop an STC to fit it, change the engine instruments, engine mount, possibly cowls, and whatever else is needed...
There is a diesel 182 parked opposite our workshop belonging to somebody else, its been sitting three or more years now, no work for it, nobody interested in buying it, so it sits there...