Because MySQL has chosen the wrong index.
You can visit this article about how MySQL selects the wrong index. Here is the link below:
https://blog.actorsfit.com/a?ID=01300-33e9ffff-ebbb-426a-8bc1-54792f144e6a
But this article is plagiarized from someone else, and the original link is https://time.geekbang.org/column/article/71173, I hope this may helpsolve ur problem!