"Most likely explanation" doesn't cut it. The egregious problem in all of this is that there is a Constitutional requirement laid out and absolutely no way to enforce it.
Regardless of Obama's natural-born citizenship (or lack thereof), the simple fact that there is no recourse to verify this most basic requirement for the Presidency is a scandal in and of itself.
With that I agree with you. The founders wrote the Presidential qualifications into the Constitution, but didn’t enact any measures to assure that only a qualified candidate would be elected by the Electoral College, or even any measures to check the qualifications of either a candidate or the electoral victor. We need to enact those measures so that no unqualified person ever runs or is elected going forward.