Because it's end-to-end encrypted for just the people in the chat. Slack and Hipchat store the logs in the clear for themselves and the contents could be revealed if they were ever hacked. Since they're also centralized this poses an issue because they're an easier target than say a member of the chat's device.

Since keybase is all encrypted, signed, and authenticated the fact it's centralized only matters as far as the service going down, but the content is all secure.