Ease of use shouldn't be confused with ease of setup.

I use discord every day but I am not a gamer so my use case is different.

Schools had to set up classes quickly and use a platform that provided the least amount of friction for students. They likely figured out most kids were familiar with discord over zoom.

I never heard of schools using Discord but it doesn't sound like a good idea. And again, I say this as someone who likes Discord. Using an app that's primarily for gamers and adding school to it sounds like a recipe for disaster.