In order to lend digital titles, libraries must abide by publisher lending policies. A common lending policy is called the One-Copy, One-User model. Under this model, libraries agree to pay for each digital copy of a title (just like the hard copies they circulate), and only one person has access to each copy at a time. This means that if you want to borrow a One-Copy, One-User title whose copies are all checked out, you have to put it on hold and wait for it to become available.
If you'd like to borrow a title that has a long waiting list, let your library know. They may be able to purchase more copies of in-demand titles.