Harashima and Madara used it for some reason at least.
And none of their successors do.
Literally, the younger the shinobi generation, the less obvious armor they wear, with only the oldies wearing their vintage shit.
That suggests one of two things:
1) The armor Hashirama et al wore is too expensive for modern shinobi.
Unlikely, if even elite jounin and Kages alike do not wear it. Sarutobi wore less than his teachers(a helmet , mesh and vambraces).
Jiraiya only wore mess underneath his clothes. Minato never wore anything heavier than a flak jacket. Neither did Kakashi.
All could probably afford the very best.
2)Flak jackets and ninjutsu techniques are capable of providing equivalent or superior performance, hence obsoleting it.
Or that modern techniques are generally deadly enough to punch through any armor.
Which would parallel what has happened with RL weapons and armor tech, with improving knowledge and techniques obsoleting old procedures.
Noone bothers armoring ships any longer IRL.
Well, the fact that, say, Kakashi could block Zabuza's fucking huge sword with a kunai lends some creedance to the idea that armour could work. It just might not be worth the effort for most, though.
I assume weapons are subject to the same passive effect that allows Gai to punch a mook Sound shinobi literally through a wall without instantaneously vaporizing him.
We also see the Land of Iron make extensive usage of heavier armor, but that's probably because it's the Land of Iron, so they likely have access to extensive metal deposits with which to easily work into armor, and more advanced metallurgy with which to make said armor.
Or, because samurai use neither ninjutsu nor fuinjutsu?
They are restricted to methods that the shinobi villages have long since abandoned as obsolete.