Much more demand for Sat - Sat trips for leisure destinations than other days of the week. The flights are often sold as part of hotel package deals. enilria also noted this change on his weekly OAG update on airliners.net. Given the change was just loaded, it's hardly surprising notifications hadn't been sent yet.
I guess it make sense given that they likely have slack on Saturdays since its a quieter travel day, but either way, it's such a worthless route now that I'd rather they just ax it than pretend they have the route.
A once a week route means that if a passenger misses their flight for whatever reason, it will be a long, difficult slog to get home (I guess their best option short of paying a walk-up fare for another airline is to get up to Cancun and fly out of there?). There's literally no way to get to Cozumel any other day of the week on a Delta or Delta partner flight - even Aeromexico does't fly to CZM from MEX.
Re: notifications.. unless there's excessive call volume due to IRROPS, why wouldn't they just notify as soon as the changes are made (or at least throughout the week) with priority given to those with status?
I really don't envy anyone that has a prepaid hotel reservation.