The problem is that Microsoft copied Facebook's Messenger rather than copy Slack.. . and Messenger is a Social app, not a business focused app.
For social app, you want on to the conversation that are active now. if you miss some gossip, that's totally acceptable.
For biz app, like many people said here, I can have hundreds of conversations, with groups or people, but these are neatly divided into handful of projects or categories.
Unlike social, in biz app, missing communication is not acceptable.
Another problem that plague the SW industry, is really giving up any hierarchy. I think Google started it, but at least they gave a way to create hierarchy via tags. but all the copycats, copied only the "clean look" of no hierarchy.
The problem of organizing stuff in a hierarchy, sometimes multiple hierarchies with tags and all - was solved many times. it may not be a simple database structure, but definitely not rocket science. So I claim this is not due to lack of technical ability but really due to laziness in both product management - realizing the actual need of the actual customers, or of engineers, too lazy to go the extra mile to deal with a little more sophisticated data structure. flat is easy. so it "Hello World".