Big Bang Theory is not on Netflix in the United States but Seasons 1-8, as of late 2016, appeared on Netflix United Kingdom or Netflix UK. In the UK, new seasons of Big Bang Theory are slated to be posted on the streaming platform every September, with Season 9 coming in September '17, and Season 10 in September 2018. Other countries that use Netflix also have streaming access to the show like Germany, Ireland, Japan, Spain, and France.
Why doesn't CBS allow Big Bang Theory on Netflix in the United States? Well, the answer to that question is pretty simple: money. With the show reaching all-time ratings in the last 2 years, CBS has been capitalizing on the demand that the show has created by launching their own streaming application/stream: CBS All Access.
However, CBS hasn't kept the show under their wing, exclusively. In early 2017, the television studio inked a deal with streaming company Hulu to give users access to Big Bang Theory through the popular streaming service. Yet, it's possible that the 2 new Big Bang Theory spin-off series "Disjointed" and "Young Sheldon" will be made available on Netflix US instantly when they begin airing in 2018. Money motivates everything in the entertainment industry, so until the show loses ratings or goes off the air, it's not likely that US Netflix users will see the show on the Netflix streaming service anytime soon.