Because 6 is playing craps and he's afraid to role a 7.

Q: Why was 6 afraid of 7?

A: Because 6 is playing craps and he's afraid to role a 7.

Ah... interesting... this is the first time since this blog began that both 6 and 7 are not being personified. 6 is, but 7 is not. In this case it plays against your expectations that 6 and 7 will be personified, because in the end it is a man presumably named 6 who is in Vegas, Atlantic City or some Indian reservation and would simply be enraged if he rolled a seven and lost all of his money... or at least the money he had on this bet.

It brings up a lot of questions: what did he bet, why is the man's name 6, and why would this ever be the punchline to this joke... but it is interesting none-the-less.