Yeah, wondering the same and couldn't get it to change. Mine is defaulted to Trending last 24h.
Discussion
Can click on your profile pic, settings, feed, to get rid of "trending 24hr" and anything else you don't like (except for chronological).
won't let me reorder or remove either
Yeah, it was a solution for stopping the "Trending 24hr as the default" problem. The fact that you can't rearrange to pick the default... I hope someone is working on that.