Many 135 jobs are Single Pilot (autopilot in liew of SIC for pax operations). Hauling boxes in a Caravan by oneself still requires someone who will ACT as a PIC not just sit in the left seat. In my experience, companies want someone at or near ATP and who is independent and good enough to run a line without much home office assistance: somebody who can get the job done.
I know of a very few B1900's which HAD an autopilot installed. They were used part 135, SINGLE PILOT. Part 121 REQUIRES two pilots. (please understand my next statement is pragmatic, not intended to be rude) The insurance agents see an SIC on a B1900 flying part 121 as being there to replace an autopilot and to satisfy the FAA.
Single pilot is a different world. Amungst the usual ILS, LOC, LOC BC, VOR, my current employer (an air ambulance company) is authorized for NDB circle to land, contact aproaches, smudge pots/torches in liew of installed runway lights, and even IFR in uncontrolled airspace. It may only be a twin Cessna, but it still takes a Pilot.