Having a restricted number of books to 'lend' by a library is a flawed model unless you consider that some, unwilling to wait, will pop over to their favorite ebookstore and buy it.
Generally, though, if I want something I check the library first, and if it is available I'll wait. Even Cleveland's library though, at around 8,800 ebooks , has far fewer than fictionwise. And included in the Cleveland number are all categories, including many I have no interest in.
So the limit would suggest the ebook publishers are trying to get more money directly from impatient people with money to spend rather than license/lending fees from the library. One reason for library limitations may be so the library doesn't have overwhelming fees in a single month for popular authors - limiting access would suggest monthly or quarterly lending/licensing fees would be at a predictable level.