Why Didn't Old Biff...

Bad memory?

Maybe he did, he just didn't care.

Old Biff, when was in 1955, seemed amused by the memory of the manure truck. He laughed as he remembered it. Almost as if it had been a long forgotten memory by this point. Maybe he just figured that making himself the richest man in the world was a higher priority than worrying about something embarrassing that happened to him 60 years previously.

Maybe Biff just wasn't that clever. He did go round saying; "Make like a tree and get out of here."

1)Biff isn't very smart.

2) He probably knew Marty since he was a baby, and just didn't notice that he grew up to look like Calvin/Marty.

3) That's probably the worst week in Biff's life and he's probably tried to forget it.

4) And when he does think about it, his first thought is getting laid out by George.

5) He was an old man, who probably didn't take good care of himself. Most likely his memory might be a little spotty at that point, especially over something that happened 60 years ago.

Eye witness testimony is some of the most unreliable. I know Biff met Marty multiple times but no way he would remember a face 30 years later he saw 4 times and logically couldn't think they were possibly connect.

he didn't know old biff was himself. I dont think he knew time travel was involved. he even says he was shocked and never thought Marty would be the one who would ask him about the almanac