I was thinking about the scene where Doc and Marty see the cops taking away Jennifer, Doc tells Marty they can't clear things up with the cops, remarking "what do we do, tell them that we're time travelers? They'd have us committed."
Now that is a good point, except that they do have proof of a time traveling device. They could have told the cops, showed them their time machine, sent them back in time, and they'd prove it.
Though it also seems like Doc was trying to keep the time machine a secret. I wonder if he had always intended on his time machine for his own personal enjoyment (even if he did have Marty record information about it), or if he had originally intended on marketing the time machine, which I wouldn't be surprised if he'd changed his mind afterwards, after realizing that it would be bad if it fell into the wrong hands (or even after 1955 Doc realized that it's better not to know too much about the future). Of course, Doc's 1955 meeting with Marty most likely had something to do with him inventing it as well, since Marty would need to go back in time and so on.