There is no way to ever know the extent of why, these are just theories.
They aren't even theories. Theories can be disproven. The are simply your personal myths.
I'd say the likely reason is that, for somebody in the demographic that owns a PC capable of running Skyrim and has the expendable income to purchase it, completing the main quest takes a pretty low priority compared to other requirements and interests in their real life. I'm surprised those numbers are as HIGH as they are.
Consider my wife. She loves the [censored] out of this game. And yesterday she asked whether I'd finished it, given my 250 hours of play. I said no, and she was happy because (her words) "I want to be able to keep playing this game for the next five years without getting board, so I don't have to buy another game until I buy a new computer." (Her computer was brand new for x-mass).
People like me and my wife don't buy a game because we want to finish it; we buy it because, like a good piece of music, we want to have it around as entertainment to play when we happen to have (or in my case make) the time.
Here's a stat you didn't mention. Average play time for PC users as given by Todd at DICE- 78 hours. I (like just about any forum user) am probably a couple of standard deviations on the high side of that, but that's still a whacking huge number. It means the average PC user spent about an hour EVERY DAY playing the game at that point, since the day of its release. And of course, not all of them bought it at release, meaning they had to pack in more minute per day of play to keep that figure up.
For a demographic like mine, that's an AMAZING level of game usage.