It's probably raining there right now (or soon) Rain tomorrow (Friday) and Saturday too.
Besides those mentioned, these rides can close as well: Tea Cups, Small World (though not often), Matterhorn, Storybookland Canal Boats, there maybe others at DCA. I know I've been on Screamin' when it's raining so they don't always close it. BTW rain hurts when you're on Screamin"

The difference between whether a ride closes or not depends on if it's just sprinking, or if it's pouring.
We used to schedule going to DL (before it was DLR) on rainy days-no crowds!
If it rains, there are some good options to keep dry. At DLR you can go into Innoventions-you can tryout a Segway (for free). Kids can do stuff as well. Ride the Monorail.
At DCA a great place to get out of the rain is Disney Animation Building. You can spend several hours there doing Animation Academy (do it a couple of times even), Turtle Talk w/ Crush, and Sorcerer's Workshop. It's a great place to go, not usually very crowded, and if you do the Animation Academy you'll have a wonderul souvenir to keep (your own personal sketch).
If it's near meal time eating is a great way to get out of the rain.