One, Biden is a moron of astronomical dimensions. Even after Hillary's emails - it didn't occur him to him to check, recheck, and triple check to be 100% certain he had no skeletons in his closet related to secret material. Or aides suggested he do the checks and - for whatever reasons - he lied and said such checks had already been made.
Two, choose your conspiracy theory. I like the idea that he and his team monumentally fucked up didn't properly vet members of the Secret Service charged with protecting him. They were loyal to Trump so they royally fucked him.
I think there two categories of answers.
One, Biden is a moron of astronomical dimensions. Even after Hillary's emails - it didn't occur him to him to check, recheck, and triple check to be 100% certain he had no skeletons in his closet related to secret material. Or aides suggested he do the checks and - for whatever reasons - he lied and said such checks had already been made.
Two, choose your conspiracy theory. I like the idea that he and his team monumentally fucked up didn't properly vet members of the Secret Service charged with protecting him. They were loyal to Trump so they royally fucked him.