Because it's /Panem/ that's why. You're talking about a country that rules via suppression, poverty, and cold blooded child slaughter. Higher standards of living for the Districts mean that they have more time (so to speak since they are not preoccupied with basic survival) to think about the brutality of the Games and how they shouldn't exist at all in the first place. It leaves too much freedom and time for the people to question the Capitol's power, or more accurately, Snow's.

Actually, if you think about it, the Districts aren't mentioned to have much high-tech equipment and all that, except for what they need to produce their Districts produce. They don't have much available to them really, on a personal usage level. The Capitol is the central place where you see all the classy technology. And isn't it true that the Capitol has a much higher quality of life compared to the Districts? All the glitz, glamour and glut.

As to coal, actually I was somewhat surprised that there was no mention of nuclear power being used...