The ADHD brain is incapable of anticipating and planning for the future, which shows up in two ways: people with ADHD frequently have a very narrow "time horizon," and they participate in "temporal discounting."

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The economic term "temporal discounting" illustrates this truth: the further into the future a reward or penalty is, the less attention we give to it now. If you were offered $100 to shovel a snowy driveway, you might take it if the money was paid right away. However, if payment is delayed by three months, the prize becomes far less appealing, making it less likely that you will agree to perform all of the shoveling today. So basically if there is no immediate reward for the ADHD brain, then the task is going to get less attention, if not forgotten altogether.