Just guessing here, but they have a product that promises much more than what it currently delivers.
They are the personification of how the cloud facilitate the life of the final user (not a buzz word that no mainstream media really understand or that is only helpfull for a very restrict niche of developers). And when you know that they are expanding to other services (as acquiring Mailbox) you can easily imagine that the sky is the limit. They might just become the next Google. And entering Google when it still wasn't Google is much better than entering Google right now.
And I think the acid test for them was when Google and Microsoft launched their direct competitors. Passing the "what if Google launch a product to compete with mine" test is like equate with them.