There'd possibly be problems with the variability of the coffee beans/instant coffee (as it probably varies in whatever compounds season to season like most plant materials) and people expect reliable results from a storebought developer, as opposed to them mucking about with stuff at home. Testing and the like would make the price go up which would put off some people. And the other appeal of caffenol is that you can make it from stuff bought at the grocery store, you don't need to go to a photographic supply place to get it.