Usually, I want to know reasons... but it's in the title of the thread. The list I could make of books that I never finished because I found them incredibly tedious would be quite long and littered with the names Hardy, Dickens, Bronte, and a hundred fantasy authors.
I think the reason should be why you continued to read all the way to the end when you found the book so boring.
Tolstoy's
War and Peace. I read it because it was late 1990 and we were on the eve of the First Gulf War.
Jordan's
The Eye of the World. Because I was sick of posting, "I've not read it, but it probably sucks." Now I can post, "It sucks."
Eragon. I read it because a friend recommended it (the first fantasy he'd ever read) and I wanted him to read
A Game of Thrones. He did not read it, he watched the show.
@Wyrmlord If it was not for my book club, I would not have finished
Seveneves. It read more like Tom Clancy... some intrigue, an uneccessary explanation of the docking systems on a nuclear submarine, a bit of plot, a huge explanation of the command and control systems of the US Air Force, some intrigue, a massive and overly detailed explanation of the firing systems on 1990s soviet tanks... Clancy's subjects are actual, Stephenson's are mostly hypothetical.