Rain doesn't fill cauldrons. That's simply it.

No matter how long I wait, or how high the randomTickSpeed is, the cauldron doesn't not get filled with water in any circumstance of weather.

In the screenshot below, I have already waited at least 30 minutes with the randomTickSpeed set to 200, waiting for the command block to activate. In fact, I am still waiting as I type this, right now. It's never going to happen.

And I know that this is a real feature, because it's in the update log. You can find it here for proof of existence:
http://minecraft.gamepedia.com/12w22a#Changes

Fix it please! This feature is so useful for rain detection with redstone, but it doesn't work!

EDIT:
But if it actually DOES work, and it literally takes longer than the 30+ minutes that I waited for the cauldron to collect water, then PLEASE rise the speed by a ton. It is WAY too slow, if it isn't actually just broken, like I assumed. Because as you can see in the below screenshots, someone else has collected water via rain, so just raise the speed, and make it so that randomTickSpeed affects cauldron rain collection speed.