The offering of a show(s) (entire series or particular episodes) and movie(s) for streaming is governed by specific licensing deal (distribution rights) with the content providers, be it a channel (in case of series) or a production house/studio (in case of movies).
Netflix, enters into agreement with the content providers, which has specific clauses about availability of a content through different mediums (Disc/Streaming), along with time period, for which the agreement stands valid. The content providers, can enter into agreements of varying nature for different content - some content might be available for short period of time, after a certain cooling period (Netflix can offer a certain content, only after n months, the content goes off the air).
The licensing deals are always time bound. If you happen to see a certain content, all the time on Netflix, it might mean that the agreement was done for a longer duration OR the agreement was renewed. Some content providers charge heavily for their content to be streamed; or to provide exclusive rights to a certain vendor (there is a bidding for licensing rights for streaming, like all normal things)
For instance, back in Sep 2011, Starz announced that, they will end the licensing agreement with Netflix, as the contract renewal was ended. This meant that content from Starz, was no longer available to Netflix customers from midnight of 2/28/2012. Similarly, in September 2012, Netflix lost almost 800 hours of content from A&E, due to complicated negotiations, which in turn led to contract expiration.
Netflix would love to increase the library of streamed content. It (and its competitors) must be working hard to win more and more content deals, but at times, it is limited by the value of the content license.
Reed Hastings, said this in an interview:Source
"We need more money in the ecosystem driving money to content," he
said. "Now we are an active bidder. It drives pricing for output up --
which is great for content producers."