While That '70s Show has been off the air since 2006, it continues to a popular viewing choice for many people, which is why you can currently watch it on Netflix.
The title was set to expire on October 1st, and even reportedly disappeared from the service for a short time. It's on there now, however, indicating that Netflix renewed whatever deal it had in place to make it available to customers.
Netflix typically lets these deals it has with content providers expire if the titles aren't being viewed much, but it would seem that plenty of people have been viewing That '70s Show. Otherwise this discussion would be a lot different.
Having the show available, however, could also have a promotional effect on Netflix's own original series Orange is the New Black, considering that Laura Prepon (the most popular cast member on OITNB, based on search data), stars on both shows. Likewise, Orange is the New Black's popularity could further fuel viewings of That '70s Show.
Here are some more recent titles that Netflix has added.
Hat tip to What's On Netflix