It's a business strategy
Netflix’s success is dependent upon its ability to keep its subscription costs from skyrocketing (although, turns out they're rising anyway). So it has to carefully weigh how much it's willing to pay to acquire new titles with how many people are actually willing to watch them. In the words of Netflix’s Jenny McCabe in a video released by the company in 2013, they have to focus on functioning as an “expert programmer” rather than just a distributor of everything.
Movie studios try to keep the most popular stuff off Netflix
You’ve probably noticed it takes a while for the big blockbusters and Oscar winners to make it to the pages of Netflix. Well, you can blame the studios behind those big, important films, who work hard to squeeze as much cash from them as possible -- keeping them in theaters for months, then releasing DVDs and Blu-rays, then selling digital downloads via iTunes and Amazon. Only then do they hand over streaming rights, which even then can be prohibitively expensive.