On election day Trump was on his way to an easy victory, then they suddenly stopped the counting for a few hours, which has never happened before, and then suddenly his large lead in several states started evaporating, and then overtaken.
So to answer your questions, they probably needed time to get the fake votes.
First the election process took so long this time. Secondly the postal mail votes changed the course of election from trump to Biden and trump is claiming these votes are fake and they were used on purpose to give him a loss. If trump did not agree on the results, this election process may extend even further.