Easy to say Slack, JIRA, Trello suck, but what are the alternatives? I used IRC at two 1000 person companies. It was useful, but didn't replace email like slack does now. Likely due to lack of persistence, absence on phones and hard to setup integrations.
Echoing a lot of the comments, you can blame tools for altering the culture. Phones disrupted memos, email ended vacuum tube delivery, computers killed the file cabinet. Slack is a reflection of our short attention spans. No one wants to read long emails. People need an easy way to message and get notified on all screens. People pass screen shots across their devices. It solves problems. I've seen people who abuse @mentions and @here, but ultimately you develop filters and need to take responsibility for one's own workflow.
I do hear how Slack isn't good at making organizational events searchable and good at archiving in corporate memory. Problem space is: need to search, recall, store and archive better. Maybe tools to record this into some record or doc, (i.e. readthedocs). You can argue Facebook/Yammer style status posts is a better way to broadcast and save announcements. Slack can innovate on UX, but they seeme focused on growth and quality of service. They're not looking to solve small issues at this time.
It's viral business model, phone/desktop apps, and integrations have made it a leader in workplace messaging. Hooks you with free and hits the sweet spot of UX.