Because Fri & Sat are more popular check in dates and Disney has no incentive to offer a discount if rooms fill up at the rack rate.
In my experience unless you see availability online, calling won't help. Another option is to do a split stay, make 2 separate reservations so take advantage of the discount for part of your trip. I just did that for my trip, I was booked Sat - Sat and after checking for over a month the discount was suddenly available Wed-Sat.