Sauron could not fathom anyone being able to withstand the power
It is stated in the Council of Elrond, as other methods of disposing of the ring are brought up, when Gandalf makes a point that Sauron would never think of anyone wanting to destroy the ring, because of the great power it possesses:
"Into his heart the thought will not enter that any will refuse it, that having the Ring we may seek to destroy it. If we seek this, we shall put him out of reckoning."
The Fellowship of the Ring - The Council of Elrond
And Sauron was right: even for Frodo, who had borne the ring countless leagues, the temptation in the end was too great just before its destruction, and without the help of Gollum the Fellowship would not have been successful.
‘I have come,’ [Frodo] said. ‘But I do not choose to do what I came to do. I will not do this deed. The Ring is mine!’ And suddenly, as he set it on his finger, he vanished from Sam’s sight.
The Return of the King
Frodo is shown giving in to the temptation of the ring when it comes to his final hurdle, and he submits to its power. It was however his initial resilience and Sauron’s mistaken thought that led him to leaving Mount Doom unguarded.
Finally as excellently pointed out by @Pryftan in his answer found here, Tolkien writes, in the famed Letter 131, that Sauron did not need to wield the ring to hold his power unless it was claimed by someone else. It goes on to describe how he was convinced that no one could possibly destroy the ring before succumbing to it. (Which we've shown above to be true).
But to achieve this he had been obliged to let a great part of his own inherent power (a frequent and very significant motive in myth and fairy-story) pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'. Unless some other seized it and became possessed of it. If that happened, the new possessor could (if sufficiently strong and heroic by nature) challenge Sauron, become master of all that he had learned or done since the making of the One Ring, and so overthrow him and usurp his place. This was the essential weakness he had introduced into his situation in his effort (largely unsuccessful) to enslave the Elves, and his desire to establish control over the minds and wills of his servants. There was another weakness: if the One Ring was actually unmade, annihilated, then its power would be dissolved, Sauron's own being would be diminished to vanishing point, and he would be reduced to a shadow, a mere memory of malicious will. But that he never contemplated nor feared. The Ring was unbreakable by any smithcraft less than his own. It was indissoluble in any fire, save the undying subterranean fire where it was made - and that was unapproachable, in Mordor. Also so great was the Ring's power of lust, that anyone who used it became mastered by it; it was beyond the strength of any will (even his own) to injure it, cast it away, or neglect it. So he thought. It was in any case on his finger.
The Letter of J.R.R. Tolkien - Letter 131, to Milton Waldman
Tolkien clearly outlines that Sauron was unable to realise that anyone would get through Mordor, up the side of Orodruin and into the Sammath Naur without having given in to the temptations of the ring. This is most likely because he had never met Hobbits nor was able to foresee their lack of desire for power and their content for a simple life.
Again, as @Pryftan states, it is speculated that Gollum's "slip" was one of the interruptions by Eru Iluvatar (the Almighty) in the series which ended up destroying the One Ring.