You did not have to cancel. You chose to cancel.
You can't have your cake and eat it too. One thing you like is advanced schedule but then you cancelled. If you are going to sign up for job to lock in some days you should honor.
Cancel because you got a better contract is not a valid reason in their mind. It does not need to be in the policy manual you received.
Apologize and tell them you realize it was wrong to not meet your commitment and ask for another chance.
When you cancelled you should have considered a possible consequence was them taking you off the schedule.
Understand if everyone would cancel for better pay it would kill them.