For the same reason a manager isn't going to value a $10MM grant in company that ended up being a penny stock, he isn't going to downgrade a company that resurrected from the pits either. Money in hand last year is the best measure possible/available.
edit: and then people will come on here and say "look! Brand new Google employees make 200k!"
Looking at how they handle Google, for example they show base|annual stock|signing bonus.
So when you see 130K base + 33K stock, that was a grant of 130K vesting over 4 years. And then with signing and annual bonus of ~20K, that person is in fact making 200K starting (although the median is probably a bit lower for a completely new employee).
On the other hand, people often forget about refreshes when looking at these things. And while a refresh grant won't be as large as the initial grant, they do make the overall comp significantly larger.
In later years the situation is more complicated as the stock price will move, you'll receive annual refreshes which are performance dependent, and you might reach a new level. While the base salary changes with levels can be modest, 15% or so, refresher targets can practically double from level to level.