Mearls wrote an article about Challenge Ratings that actually specifically explains the Ogre's Challenge Rating of 2:

http://dnd.wizards.com/articles/features/building-adventures-0

The important bit for your question is:

For example, an ogre is worth 450 XP and is a CR 2 threat. A party of five 1st-level characters should expect to face about 500 XP worth of monsters for a challenging fight. Sounds like the ogre is a good fit, right?

 

However, the ogre is a CR 2 creature because its damage is enough to drop most 1st-level characters in a single hit. That doesn't mean you shouldn't ever throw an ogre at a 1st-level party. (One of the most memorable games I played in featured a running battle between 1st-level characters and an ogre.) But it does mean you should be ready for a tough fight—one that requires a mix of good luck and smart play for the heroes to emerge victorious.

So the answer to your question is that the Ogre's CR is 2 because it is capable of killing a level 1 character in a single hit.

Mearls wrote an article about Challenge Ratings that actually specifically explains the Ogre's Challenge Rating of 2:

http://dnd.wizards.com/articles/features/building-adventures-0

The important bit for your question is:

For example, an ogre is worth 450 XP and is a CR 2 threat. A party of five 1st-level characters should expect to face about 500 XP worth of monsters for a challenging fight. Sounds like the ogre is a good fit, right?

However, the ogre is a CR 2 creature because its damage is enough to drop most 1st-level characters in a single hit. That doesn't mean you shouldn't ever throw an ogre at a 1st-level party. (One of the most memorable games I played in featured a running battle between 1st-level characters and an ogre.) But it does mean you should be ready for a tough fight—one that requires a mix of good luck and smart play for the heroes to emerge victorious.

So the answer to your question is that the Ogre's CR is 2 because it is capable of killing a level 1 character in a single hit.