Because in 2016 all he was was TALK. There was no track record of being president to check. Hot air blowhard.
In 2020 however there is an EXTENSIVE record of what the trump has done or not done or left undone that people not only are aware of but have EXPERIENCED. That is what is terrifying him.
Using mail-in voting as a ruse to excuse the enormous loss he might well face is what he does. What he always does. What he cannot help but do. His MO. Predictable. Bankable.