first of all, it would be good to ask whether they want to spend their free time on literature at all :) i think that those who want to always find time for that. but, generally speaking, students lack spare time because they have a lot of homework; besides that, others have jobs and other activities.
lack of time
too much homework and compulsory reading
most often they have enough other work. but some students are really fast and manage to find some time for reading if they are keen on it :)
while studying english philology, i do not have enough free time for reading the literature that i like.
homework, being exhausted.
studies that require hard work leave very little free time.
in my opinion, the main reason is the lack of time.
the question is whether students have enough free time at all. as for me, my studies take up too much of my free time.