2) It's written a little "higher" than modern popular young-adult-influenced fiction, mostly in that it sets up tension where the reader shouldn't know what's going on, then resolves it somewhat later, and relies on the reader holding context for a sentence or two for a thought or idea or image to resolve. This used to how books even fifth graders were expected to read were written (i.e. this wouldn't have been regarded as challenging) but extreme preferences for ease-of-reading has driven shifts in fashions (as publishers desperately seek a market in a world where readership is already very low and still declining) and now lots of folks find once-easy works of fiction "hard" (even, and perhaps especially, native speakers) simply due to rarely encountering fiction that's not, as it were, pre-chewed, so not exercising those reading skills.
3) It's written in second-person, which is a bit unusual. (First-person also used to be rare, but has become common because young readers find it easier, and it is now taking over popular fiction—I expect narrative perspectives other than first-person are the next thing that will be regarded as "difficult" by the folks just becoming adults now, and know for a fact that many current teens and tweens are expressing that to teachers, that they find reading narratives in 3rd person difficult and so mostly just... don't read those)
[EDIT] I'm not asserting that 2 or 3 are the reasons you specifically are having trouble with it, but pointing out a couple observations about it that might be reasons someone would have trouble with it. There could be others.