> Students are the only ones who want to change it
Your statement is incorrect. Students don't care about Blackboard because their interactions with it are minimal. They just submit stuff and use it to view slides, videos and grades. For this it works well enough.
I was a student user of Blackboard for a while before I saw the teaching side of it and I didn't have any negative opinions of it at all at that time. I did think the UI was a little clunky and dated but that was it. As a TA, I now realize it has lots problems. For instance, right now, I am trying to create a custom grade that is a weighted sum of other grades. And oh boy, what a fucking pain something that should be straightforward is turning out to be!
> Professors don't care. Administrators don't care.
Professors and admins very much do care about providing a good learning experience. Why do you think Piazza has taken off in such a big way? AFAICT we do our best to use the best possible software solutions. Others have already touched upon why such systems are difficult to change, but change is definitely possible and is already happening.
At my university, I am seeing more and more stuff moving away from Blackboard and into Piazza and other custom solutions. Many CS courses use a custom homework submission system that enables scriptable grading. Course material is moving from being hosted on Blackboard to Piazza. This is despite the fact that Piazza's course page function kinda sucks and provides no real way of organizing slides and handouts. But then neither does blackboard and it's helpful to have the slides on the same places as where the questions come from. There are courses where Blackboard is only used to keep track of grades. I believe this wasn't true just a few years ago and I suspect there are university regulations which necessitate this.
The lesson I learned from Piazza succeeding is that if you make your tool actually useful and really easy to get started, professors will use it. And the success of Piazza directly contradicts what you're claiming.