If there are more than two thousand years between Isildur cutting the ring from Sauron's hand and Frodo's mission to destroy the Ring, why was there no technological progress? E.g. Saruman had some kind of gunpowder and they could easily have made stronger weapons from it...
A major theme of Tolkien's Middle Earth is the gradual decline from the near-perfection of Eru's original conception of Arda. The timeline for introduction of major new pieces of Maian/Elvish technology adheres to this. Works of the First Age cannot be reproduced in later Ages; even the works of men in the Second Age were superior to those of the Third Age. The clearest example is the decline from Two Trees to the Silmarils to more mundane illumination.
Works of the Great Enemy, on the other hand, run in the opposite direction. The agenda of Melkor/Morgoth/Sauron lay in the realm of moral suasion and the establishment of an opposition power base. So far as we know, they focused on technology only incidentally and not because it was an "advance". Introduction of the rings of power was progress of a sort, a technological advance but a moral decline.
Getting back to your specific question about Saruman - gunpowder was a minor thing by itself. His larger introduction of technology in the sense we know it, the great machines and spawning vats of Orthanc, the stereotypically grim industrial conversion of the pastoral Shire, was clearly portrayed as an act of corruption rather than one of improvement.
To the extent that Tolkien viewed technological progress as a good thing, it would have been contrary to the story of Arda Marred. So he left it out. To the extent he viewed technological progress with great suspicion, he showed it as a tool of the enemy.