I'm not sure why most of the answers/comments talk about airliners as the question did not specifically asked about them, but about civil aircraft in general.
Is there a particular reason why there aren't (almost) any single-engine jet powered aircraft?
I'm going to assume that the term "jet powered aircraft" refer to turbojet or turbofan engines and not to turboprops as there are plenty of single-engined turboprop aircraft.
Having a single jet engine is challenging as there are very few feasible locations for engine placement, each comes with its own issues.
If you put the engine inside the fuselage, it means it needs to be in the rear.
This means the entire central fuselage must be entirely or almost entirely hollow to allow for the air intake/s. This of course does not leave a lot of room for passengers nor cargo.
If you put the engine above the fuselage:
The front part of the fuselage might block/shadow the air flow to the engine in higher angles of attacks.
It is probably going to be a maintenance challenge to reach the engine.
Cabin noise insulation needs to be very good when you have a jet engine a few inches above your head.
Most likely requires a V-tail (or H-tail) if you don't want the vertical stabilizer inside the engine exhaust.
Turboprop engines are much more efficient in the flight regimes applicable for a small civil single-engined aircraft than "pure" jet engines.