Probably because blockchain.info is just a wallet whereas coinbase actually allows you to buy and sell bitcoins. Exchanging one currency for another (where one is only loosely labeled a currency by the government) is where you get into sketchy regulatory waters. I imagine these are the types of problems apple would like to avoid.
Update: On February 5th, 2014, Apple removed the Blockchain.info app from the app store.
Apple has removed the world’s most popular bitcoin wallet from its App Store.
Late Wednesday, the tech giant yanked the Blockchain app from the popular mobile app marketplace and did not provide a specific reason, says Nicolas Cary, the CEO of Blockchain.