I got the coronavirus so I decided to visit my grandmother at her house since our relationship was frayed
I think you should have just stayed at home. Why did you visit your grandmother?
I wanted to know why she hasn't been visiting me.
That is very irresponsible for you to do something like that. If you are sick you shouldn't be visiting elderly people or anyone else. You don't want your grandmother getting sick or ending up in the hospital.
BUT I WANT TO KNOW WHY
I think the reason is that there's an ongoing pandemic, the one that you currently have, which is the coronavirus, that's why she is not visiting you. I hope you be more aware of the current world events so that you do not get confused why people are doing what they're doing, as they are just making sure they're safe and staying at home during the pandemic.