The problem is, we don't all get the same offers - I just used a Hilton one, and the only new one I have is an Embassy Suites.
You got a Doubletree. And if I want a Doubletree offer, I'm SOL unless I randomly get it, too.
Everyone just has to be vigilant about watching their own accounts - it's not that difficult, you don't have to click anything, just scroll down when you're on your account page and voila!
And I don't think there is a limited amount of availability to take the offer - I think as long as you sign up for it before the expiration date you're ok - have you found that you were shut out if you waited?