According to Tomb Raider Global Manager Karl Stewart, they chose not to release a demo because they feared doing so would spoil part of the single-player experience. "There are no plans to release a demo of the game pre-launch. We don't want to spoil the story," Stewart explained on Twitter, "We've weaved such a personal story we don't believe spoiling it by having you wait a week or so to play on, delivers on our promise."
He also confirmed that the game won't have any sort of online or season pass, so make of that what you will. I for one am glad to see that online passes aren't quite as prevalent now as they have been the past couple of years.
Look for Tomb Raider when it hits stores this March.