There are certification requirements for both the aircraft and Ops Spec approvals for Part 135 (on-demand charter) operations. I can see that these would be certified to fly (and thus eligible for Part 91 (private) operations) far more easily than the more stringent requirements for Part 135 charter operations.

The FAA thinking is that you have greater understanding as a passenger on a Part 91 operation and are better able to judge the risk yourself, whereas a Part 135 (charter) or Part 121 (scheduled airline) operation, the public cannot effectively judge the safety of the operation so the FAA holds them to a higher standard.

Single engine Part 135 is possible, but there's a large amount of focus on redundancy: https://www.aviationconsumer.com/industry-news/single-engine...