Just curious - was there a longer than usual afternoon storm? In my experience, if it rains for more than 20 minutes, many people go back to their hotels until it stops. Then, when it's over, they head on back to the parks. Or even if they stay, they linger in restaurants and stores. I always had rain gear (including a cover for the stroller when my children were tiny) so continued walking around unless there was lightning close by.
I had the park to myself for a while, until people started coming back outside.