We know why! When installing the Rafflecopter Facebook app on your giveaway's installation tab and the 'Run on a Facebook Page' button is clicked, a pop up window would have asked to ‘Log in w/ Facebook’. Once clicked, a Facebook popup called “Rafflecopter Tab” would have appeared. After clicking “Log In with Facebook”, everyone must give the “Rafflecopter Tab” permission to manage your pages by clicking “Allow”.
However, if you click "Skip" instead of "Allow", the next page's drop down box won't have any of your FB pages listed because the app won't be able to connect to see them. If this sounds like what happened, here's what you can do to fix it:
Go to your giveaway's installation tab on your Rafflecopter account and click the 'Run on a Facebook Page' again. You should see this popup.
Beneath the drop down, it reads "Don't see your page(s) in the dropdown menu? Rafflecopter needs permission to manage your pages." Click on the 'needs permission' link so you can now 'Allow' Rafflecopter permissions so you can install the giveaway on your page.