Simply because they are more pleasing to the eye to me than the Johnson Controls.
I do like that the JC has the ability to have a night drop hooked up to it, other than that they're the same.
I think the Rancos click a little quieter than the JC as well, but that's inconsequential.
I pretty much feel the same way. Although I have noticed that since I switched out the thermostat on my oil-filled heater from a Ranco to a Johnson (for the night drop) I get a 3-4 degree swing with the Johnson between turning on and off as opposed to 1-2 with the Ranco.