Because 7 was falsely accused and 6 can't get over it.

Q: Why was 6 afraid of 7?
A: Because 7 was falsely accused and 6 can't get over it.

This answer has never really been explored in the world of the joke, but the answer makes sense. In this case both 6 and 7 are being personified yet again. 6 is falling into the same old trap that we humans would fall into when we are convinced of one thing, only to be confused and startled to find that that in fact is not the case. 7 was a red herring if you will, and 6 is acting as the Sherlock Holmes incapable of letting go.

This brings up the question: "What was 7 accused of?" Many answers. Many, many answers. 7 could have been accused of terrorism, murder, molestation, prostitution, grand theft auto, fraud, cheating in a fantasy football league, or all of the above. There is no limit to what 7 could be guilty of.

But of course that is irrelevant to the joke. This is simply playing on the fact that 7 was falsely accused of some thing, and 6 is falling into the all too relatable and human trap of believing the first thing he or she sees. And that is why this joke is relatable.