Why does CO "close" the waitlist for international travel weeks before departure?

I purchased a full Y on CO99's EWR-HKG on 2/17 with hopes of using mileage to upgrade (I'm a Platinum with more miles than I know what to do with.) I haven't used any online tools to check out the real availability #'s, but the seatmaps show about 10-15 open seats up front and more than 1/3 of the back empty. I know seatmaps aren't an accurate depiction of sales, but I'd figure directionally they show decent availability there.

In any event, I called for my upgrade only to get "sorry, there are no upgrade seats available and the waitlist is closed." Other than those lining up to buy regular J seats there, there shouldn't be anyone else (short of non-revers ) who's able to trump my status + fare paid. As such, shouldn't CO keep the waitlist open and bump those on the list with lower fares and/or status to the bottom of the list?

NW does have availability for roughly the same fare (althought a pitstop in NRT nets me more EQM's ) so does CO really want to lose my Y fare to them? It's one thing to hold back seats for sale for real J, it's another to annoy a top tiered elite paying a decent fare simply by adding an additional layer of complexity by waiting for a waitlist to open for real upgrades.