What prevents DC-9 series aircraft from being reengined with more-efficient engines?
A re-engine program can be pretty expensive. This can make sense on military aircraft that take much longer to accumulate hours and cycles, or even cargo aircraft like the DC-8 in a similar position.

But the newest aircraft in the MD-80 series are going on 20 years at this point. They just don't have enough life left to justify that kind of expense. Most of them are not equipped with glass cockpits. When you add on the avionics upgrades needed, it becomes more cost effective to just buy a 737 or A320 which comes with the latest engines, avionics, and other features.

You're going to need a major manufacturer willing to take on the effort of an engine replacement program. That would point to Boeing or Airbus, and that would only cut into their existing narrow body sales.