Agreed. In the general sales, Sunday badges are the last to go, but they do go, and relatively fast.
I think there are 2 big differences here: first, we are now less than a month before the Con, which is very late to make travel plans if you aren't in the local area. Second, the availability of Sunday-only badges was not advertised, so 'locals' from the LA/SD area who might be interested in a one day trip haven't necessarily heard about the badges.