Slack is not as effective for mobile devices, being a full featured desktop app, whereas Telegram is. Plus, there are already 10 witnesses using it, it's just a matter of getting the others on board.
Based on the range of missed block counts I see it's apparent there are different levels of dedication and responsiveness in the witness pool. I think you can see a unanimous consensus is not likely to happen for one particular platform if that is any indication. So the goal of ALL witnesses may not be achievable, and if it is will take time. Hopefully the other witnesses who are NOT on Telegram will see this and get on it, or at least post here with alternatives.