Mainly because Disney DID offer some recovery to the March/April guests (when they assumed/hoped this thing would only last a few weeks (probably like it should have)). I will admit .. I kept my June reservation (knowing it would be cancelled) in case they were also going to offer some sort of "come back" offer when they cancelled my reservation.
But I agree .. Disney doesn't NEED to do anything but offer a full, no strings, refund. I also don't think they really need to do anything THIS year to encourage people to come back .. at least until they feel things will be back to full "normal". When they are ready to transition to FULL capacity, I am sure they will toss out some promotions to QUICKLY fill those rooms with "last minute" vacation plans.
But I think a lot of people are disillusioned with planning ANY vacation in the near future now and are going to shift to way cheaper and more flexible vacations this summer and fall .. so eventually Disney will probably need to encourage ALL guests to come back (not just the cancelled ones)