FIrst of all, it HAS happened here.
Remember the WTC bombings in the 1990s?
If they COULD set off a bomb in Times Square, they would. They are NOT afraid of reprisals, since they are suicidal lunatics.
The main reason it has not happened is that the USA has kept up the pressure around the world. We and our allies have killed thousands of them and arrested thousands more, and many of those captured have talked. If we ease up, then we will certainly have many suicide bombers doing their thing in public over here. We may have a few no matter what we do.