Probably because the need of moderation.
If you host an instance and let people in (even if it’s a limited circle, i.E. your students) you are responsible for moderation. I think that’s something institutions back off currently.
For an mail server that’s much easier.
Many Universities already have their own dedicated subreddits that are usually moderated by a mix of faculty, staff, and students. I know of at least one sub moderated in part by the chair oftheh math department, who is as funny as they are savage.
An above-average level of shitposting goes on, sure, but it’s also a great venue for the school’s online community to engage across organizational boundaries.