My main reason for book abandoning is... it's hard to get into. Yep, slow and boring. The latest one that I've put on my "started but got bored" shelf is from a bestselling author, Emily Giffin's The One and Only. I did read about 50 pages already, I put it down because I found a more interesting one with which I was able to finish in just 3 days and also did a book review btw.
There are several other reasons too like for the book 'The Unbecoming of Mara Dyer', the start of it is too grimmi-ish for my liking. During that time, I was expecting a bit less of people dying on the start of the book but since it did, had to put it down and looked for a lighter plot and that's when I started reading Landline by Rainbow Rowell.
I guess, it depends on my mood and current life situation. I love all books, not specifically picky plus I don't permanently abandon this lovely books. I am one of those 38.1% who finishes the book no matter what.