I think 3 things are needed before a game is bought on launch.
Want
Cash
Access (I really cannot think of much better for the third one)
If I have wanted the game for ages, Have the disposable cash at hand and the internet is not playing up I will probally get release.
Or the alternitive
If I have wanted the game for ages, Have the disposable cash at hand and the way to a store is not blocked or too unpleasent I will get it on release.
I think there have been some games I got when I just happened to be there on release, monster hunter and mad world, I got a free t-shirt with it

(Probally best part of the game)