Slack launched in August 2013.

Stride was release in September 2017.

But then it was way, way too late. 4 years is an eternity. Even Microsoft Teams had come out before Stride (in early 2017).

Slack would go public less than 2 years after Hipchat/Teams launched with $400mm in revenue (FY2019). To give you a sense of Slack's scale and some perspective, the same year Slack went public - Atlassian did $1.2B in revenue.