Why do Slack thread cards flow upwards instead of downwards?

Today someone replied to a thread in Slack, but I was not focused on that particular channel, so Slack attracted my attention to the "All Threads" page on the sidebar.

To my surprise, when I opened that page, the thread my colleague had replied to appeared not at the bottom of the window, but at the top.

I was surprised because with all other Slack conversations (and most chat apps I use) all the recent action happens at the bottom.

(Additionally it looks like the reply text box might change position a little, depending on the length of the thread.)

So, why did they do it this way, instead of arranging the thread cards from top to bottom?