It's hard to answer without you providing more details, but probably because of congestion caused by network traffic. Congestion is highest during peak times, which is when most people want to stream.
What are your streaming settings?
Why does it take over an hour to stream a 24 minute program on Netflix?
If the program is streaming in HD, you may need to turn off, or pause, the Video Data Saver. If, however, the problem is being caused by the congestion maratsade mentioned, doing so may not make much of a difference, if any.