alt Hacker NewsProbably because there is demand. Could be that there was very deep waiting list at some point. Or there has been deep waiting list for specific author before. Fulfilling these demands does require multiple copies or it could take years for people to get popular book.
Sorry, I don't think popularity should be a factor in library decision-making. Extremely popular books driven by massive marketing campaigns predictably translate into the same book being available for only a few dollars months later. This all sounds like it's driven much more by the needs of publishers than library users; consider that the more reduced the selection, the fewer people will come to use the library because they can't find enough interesting material to read.
My local Half-Price Books (a second-hand bookstore chain) has a vastly better selection than my local library.