Slack is not strategically interesting. They are already eating the enterprise communication market with Teams. Acquiring Slack would basically be a talent and tech hire nothing more... not worth $8B.

Discord gives them something gamers know and love and could be paired with Xbox. Gives them a huge competitive edge against Sony, especially if they intend to build a cross-play centric platform between Windows and Xbox.

Probably passed on Slack due to anti-trust, since Teams + Slack would be owning a large majority share of group chat.