The reason of this change was because the previous logo was created before the company launched and since there are 11 colors involve then it was very difficult to make combinations with backgrounds.
As you can see, there isn’t any good consistency at all.
So, if at some point you asked why we have different versions of the logo for the Slack app, the icon and the logo of the website, the reason is the mentioned above.
Here you can see the big difference between the old and the new logo for the different Slack environments.
In my opinion, the new one looks awesome!