Z-library were in it for commercial gain (you could access a certain number of books but to get more you had to pay for a subscription). They started out as a fork of library genesis, whose mission has always been strictly non-commercial and about providing free access to everyone without limits.
Hopefully this will encourage more people to go back to the original libgen. I suspect Z-library's popularity was because of a better interface and larger collection, but I think lots of people didn't realize libgen offers all its books/papers for free without limits.