Not sure. I haven't come across 'Joy Story' being identified as a Pixar short. Maybe it's a relatively new or less popular one that I'm not familiar with.
It's possible that the team at Pixar felt that the full-length Toy Story 4 told the story completely and didn't require a short to enhance it. Or perhaps they wanted to save their creative energy for future installments or different projects altogether.
It could be that the story of Toy Story 4 didn't lend itself well to a separate short. Or perhaps Pixar had other projects in the pipeline that took priority.
One of the most popular Pixar short stories is 'Bao'. It tells a very touching story about a mother and a dumpling that comes to life. The unique cultural elements and the deep emotional connection between the characters make it extremely popular.