That happened because all powers involved thought that enough had been accomplished to justify extending the deadline rather than just giving up. In fact, tomorrow's deadline is not even the deadline to get a complete deal - just a detailed outline of how it could be done. There would then be an additional deadline to get all the details worked out - which is the end of June.
However, nearly every country's foreign minister has spoken of more having been done this last year than in all other years of the decade this has gone on. So, it would seem like there is no way they say "it's close" now -- and at 12 midnight tomorrow, turn into pumpkins. That would make no sense.
There is one main REAL cause for a deadline - the US Congress. They have agreed to give the negotiations the time until tomorrow's deadline. BUT -- luckily for us if they are very close, but not there -- the Senate just started a two week recess. This really may give them a grace period.