Why isn't more work being done on developing advanced 2-stroke internal combustion engines?

Given that a 2-stroke configuration increased power density two-fold over a 4-stroke configuration, why does it appear that efforts to develop a more fuel efficient 2-stroke engine are minimal, at best? 2-stroke engines utilizing valves rather than ports have been developed, but have never been commercialized. I'm trying to find out what are the primary obstacles/objections to developing such an engine - either gasoline or diesel.