Well, the convention is for change logs to be called "ChangeLog" and for them to have a fairly specific format.
I suppose the format you mean is the GNU one, as in https://www.gnu.org/prep/standards/html_node/Style-of-Change-Logs.html#Style...
Is it important to you that we follow that? I don't find it particularly valuable today. Markdown is more common than that format today, and it correspond to the format we're already using for `ANNOUNCE`, removing the need for writing the same text in two different styles at each release.
Not particularly; that was mostly in response to Francisco's suggestion to rename the file to CHANGELOG.