A really good question was asked over at the Straight Dope:
We keep hearing in the news about radiation levels in milk and whether they're high enough to be a concern. Your recent column about the safety of nuclear power also mentioned contaminated milk. But nobody explains why contamination is a big issue with milk but not with potatoes, chicken, or water. I always thought radiation was an equal-opportunity contaminant that lands on whatever's in its way. So what's the deal with milk?
Cecil has a lot of good answers here: milk is consumed quickly (so radioactive iodine is still active), it’s largely consumed by children, etc. For more info, I recommend the article.
He might have missed one. There’s a whole infrastructure to test milk on an ordinary basis, so we’re set up to do mass tracking and testing of milk in a way that we aren’t set up to do mass testing of, say, potatoes. Thus, milk is well positioned to serve as the mining canary of foods.