According to US laws, streamers can't stream a movie on their own without first getting a license that allows them to do so. Many Hollywood producers are reluctant to allow streamers to stream their movies.
Netflix seems to be adopting the strategy of focusing on TV series instead of movies. This has led them to produce many series while not focusing much on increasing their movie database.
Netflix wants to have original content and because of that it started producing its own series. Netflix seems more interested in exclusive movie deals than movies that can be streamed elsewhere such as on Amazon Prime. This makes the new movie options more limited to them.
It would be very expensive for Netflix to buy the rights of all the great movies. This is why they try to buy whatever can make their business better while keeping cash in mind.
Netflix is trying to save money to use it on the production of new original series and on original movie deals. In order to do that, they sometimes drop old deals that contain movies that can be seen elsewhere.
The low subscription price is a part of Netflix's strategy. In order to maintain that price, Netflix can't license everything. They have to choose certain shows and movies that bring high views compared to their licensing cost.
According to Netflix, they sometimes stop renewing the license of shows that don't bring enough viewership in order to be able to maintain viewers with the least licensing cost.
With the rise of other players in the industry such as Amazon Prime & HBO, the competition for licensing has become higher and so it became more costly to license certain movies and shows.
Many movies are licensed for a certain period of time. If Netflix got the rights to distribute those movies, they will only be available for a certain period of time until their license ends.
According to US laws, movies can be sold on DVD without licensing. So many of the good movies you want to watch are available on DVDs in Netflix but can't be streamed because of licensing laws.