From Dr. David Jeremiah's book, The Great Disappearance

We saw in the previous chapter that the Rapture is Imminent, meaning it can happen at any moment. Yet our Lord Jesus has not yet returned. For two thousand years, Christians have watched and waited for the catalyst of the Rapture to coalesce into the next phase in God’s prophetic plan. And we are still waiting.

It's fair to ask the question: Why the delay?