any number of reasons. maybe he thought he was being polite. maybe he was trying to get something from you. maybe someone dared him. maybe you are attractive, but not as attractive as the girl he hooked up with later that day. and so on.
If a guy asks you for a coffee and you say "sure" then tell him a date, if he doesnt chime in. Such as "how about this Friday, @ ----?" There you go, or can always MSG him and ask, if he'd like to crap that coffee now. If a guy forgets though, then he forgot himself, not worth a date.