the interface is much better.
search is better.
you can set the status and more details which makes it easier for people to find you and talk to you.
the group discovery is easier.
better integration with all the tools which makes it easy to set up workflows.
ability to add reminders and schedule post.
when the number of messages increase, slack has enough tools to streamline the same.
i was able to run two teams completely async using slack and other integrated tools.
bookmarks tab at the top makes it easier to find things.
ability to pin things to the top of the chat.
threads makes it easier to keep things seggragated.
document sharing, code sharing etc are much better in slack.
reactions
rich text formatting
advanced search

Making Google Chat

if your organisation doesn't agree to migrate to slack, here is how you can make Google chat or spaces work.