Let's not forget that even the 20-episode season greats had their fair share of real stinker episodes, those stinkers increasing in frequency as the series dragged on several seasons longer than it should have due to sheer momentum. And that a huge proportion of 20 episode season television series just sucked and were forgotten about.
We've also moved beyond the three-camera stage plays with very few sets to television shows where one episode is shot on more sets than Friends used for its entire run - because it makes better television. Episodes are also longer than 22 minutes now. (That's what YouTube is for.) These factors demand a lot more effort, which means there's not enough time or labor to pull together 20 episodes in a year.