The issue with Slack is that it's invaded with spammers that are uncontrollable. But rather than migrate to Discord most projects have migrated to Telegram. The issue now in telegram is every major project is littered with hundreds of speculators talking about exchange listings all day, flooding any other form of discussion. There is often no other chat for any newly released ICO project other than about exchange listings.
Why can project teams not use Discord where a Trading / Speculation channel can be created for the speculators/exchange enthusiasts, and other channels for core project discussion? Projects that use Discord are normally much better organized for their official chat channels. You can create separate channels in the server; one for trading, one for bounty hunters, one for development etc. I feel a lot of conversation is getting lost due to the sea of irrelevant posts on Telegram in every single new ICO I've participated in.
I see that, A lot of speculators or even spammers in telegram also just like slack. But Majority of people are not familiar with discord. Telegram is easy to use, and it just needs phone verification to sign up in the telegram.
I guess people are looking for easy way to communicate with the team and telegram is including on the list.
The project can manage the telegram group by give some moderators on it to control the conversation in the telegram group.
For me, discord is a bit more complicated rather than telegram.