Because sometimes your flux capacitor just isn't enough.
See templates we've used before Back to DeLorean HistoryDear John, We regret to inform you that your time traveling endeavors have been deemed "not worth it".
Our team of experts (and by experts we mean a guy in a basement) have reviewed your application and we are sorry to inform you that your flux capacitor is not up to snuff.
We've attached a copy of our standard rejection letter template for your reference. Please do not attempt to use it as a coaster, it's not made of that good of material.
Sincerely, The Time Traveler's Guide