There are effectively 3 primary reasons people don't play higher levels:Because there is a lot of problems in high levels and maybe this is the reason why many players keep their campaigns in low level tier.
- Natural attrition: aka campaigns finish their course before getting to the highest levels, and a new one begins. So by default these levels are played less. This can also be attributed to module design, which tends to be more for low and mid level play.
- Mechanical complexity/issues: These are due to balance issues arising at high levels, or the simple factor that playing at high levels is more complicated, and therefore more cumbersome for people.
- Tonal changes: High level power necessitates a certain scale of power. More superheroic saving the world or even the multiverse rather than the local town. People either don't like those types of games, or don't really want to change the tone of their original campaign, which feeds into point 1. Conversely certain Dms don't like the amount of planning and effort that often goes into these levels to compensate for the new tier of PC ability, and so again try to finish the campaign before this point.
Now the question of course is, how much does each one contribute? I don't know if WOTC has done any real depth of surveys on that, all they have done is polling on how much high level play actually occurs.