I have little insight to add in the way of specifics to the foiled Fort Dix terror plot. To paraphrase Woody Allen, we should not ask why it almost happened, we should ask why it does not happen more often.
There is an assumption that Fort Dix is so secure that no serious terrorist would dare try an attack. That may be a faulty assumption, given that Fort Dix is no longer an active duty base. And even so, one of the alleged plotters had a legitimate reason to be on the base, delivering pizzas.
In my opinion, the reason we do not see more of these attacks is that we are not going to withdraw from New Jersey. Even though there are many more "soft" targets in a given American city than there are overseas, a domestic attack would strengthen our resolve rather than weaken it.