Slack and Matrix solve different problems. Slack is "a company wants it's employees and select outside parties to communicate with themselves." Matrix is "we want everyone to communicate with each other." A broader solution is almost always less efficient for a specific constrained sub-problem.

For example, a Slack instance can be configured to log every conversation (including DMs) for audit purposes by company admins. Matrix enforces e2e encryption for peer-to-peer conversations and I believe disabling room encryption requires code changes.