Why do we not see propellers on swept wings or jet engines on straight wings?
You can find propellers on swept wing aircraft, such as the TU-95, or more recently, the Airbus A400M.

One turbofan powered aircraft that has straight wings is the Cessna Citation

No, it's not common, due largely to the speed at which the aircraft normally operates. Turboprops tend to operate at a lower speed than turbojets or turbofans, where the benefits of swept wings (better performance when approaching Mach 1) aren't evident, while the benefits of straight wings (better performance at lower speeds) comes in handy on the shorter airfields that smaller turboprops operate from. Bump the cruising speed up to Mach .8+, and the swept wing is the optimal design. 

In the case of both the TU95 and A400M, they cruise at higher speeds than most turboprop aircraft, hence the employment of a  sweep to the wings.