Because that would be forcing a rate plan change, which Koodo is not allowed to do, as it has to be initiated by the customer. If you want to go onto a higher plan, you make that decision, Koodo will not do it for you.
You can track your minutes on their app on the phone. If you think you are going to go over and you can go up a plan size. If you have an old plan though you won't be able to get it back after, just plans that they currently have.