A question about advancements of weaponry

[Tuor in Gondolin]

Insiladun has a key point here:

With the Istari gone, Elves dwindling and having over time less and less contact with the kingdoms of Men, it doesn't seem much of a stretch that over time the moral equation of impersonal technology designed to kill great numbers with "Morgothian" behavior would have fallen by the wayside. With that blockage gone, and always new enemies (of other Men), the motivation would have been there.

In Tolkien's Middle-earth universe men and mannish related peoples seem more easily corrupted then elves and dwarves. With the Istari also gone and the valar increasingly only indirectly montoring things you have people (including hobbits) more susceptible---see ted Sandyman and the shirreefs (a minority) who rather liked lording it over other hobbits, and the way men like Grima and the Dunlendings (and ewasterlings and Southrons by Sauron) were susceptible to dominant personalities and technological "improvements."