Isn't this the story of most games?
The biggest reason I feel is there are more games to play all the time.
I'm a fan of cheaper shorter games for this very reason. As much as I love some games, there is a point with many where my ADHD kicks in, the mechanics become boring and I wane.
I also would take it with some concern, as some didn't finish for reasons above or others, bottom line is they didn't finish it and that will stick with them. I feel a lot of second games in a series, with large sales drops reflects this. The customer didn't finish first game, so why but the 2nd?
Things like Blackguards and Grimrock, the follow ups where scary in drop off.