I actually think that bacterial RIs are not as common as we think. Doesn't mean that they are uncommon, just means that they should not be the first thing we assume when a snake has an RI. There are fungal causes, parasitic causes and viral causes.
In my time keeping snakes, I have had more imports come in with lungworms than bacterial RIs. When is the last time you saw someone talk about parasitic RIs on the forums? They don't because most vets just don't bother to do anything more than throw Baytril at a snake with a suspected RI.
The one time I had a spontaneous RI occur in a long term captive, it was due to a virus.
I'll repeat what I said earlier - until vets stop short-arming clients, no one knows the true breakdown of the incidences of each cause.