In the old days I’d say it’s because those weapons surely wouldn’t have survived ten thousand years anymore than a bronze swords from Egypt exist today. But sadly, the modern design studio is essentially a collection of failed comic book writers.
So swords of which we have examples from the 18th Century BCE (For example the very beautifully electrum inlaid khopesh found in Nablus and currently exhibited in the Berlin State museum), of which some have survived in better condition than iron and steel swords that were made during the 30 year war and the reformation (Such as those exhibited at the Basel Museum for History and the Klingenmuseum in Solingen), despite having been handed down through various, partially opposing cultures that might've had motivations to either purposefully destroy such objects or just simply neglect them, or having been lost beneath sands, in moors and rivers, perhaps even ice, have survived as well as relics and weapons handed down directly to chapters that persisted for at least 10 millennia, and have revered such objects more than any other possession of theirs?
I don't know friend, the only edition I can think of where this reverence and thus desire for preservation would not have been the case, is be Rogue Trader, and that is a pretty different 40k beast even when compared to 2nd edition. Sure, some relics were lost (Such as the spear or russ, which was then retrieved, I believe), but such events have been viewed as one of the greatest shames that could befall a chapter, and if boarding actions are undertaken to save a suit of terminator armour, then I can't imagine what a chapter would do to recover the Spear of Telesto.
What instead I could envision, is that such objects are not used because they have either been lost for good (Ferrus' Hammer is still in Perty's hands I believe), have fallen into disrepair due to arcane technologies and are just polished, maintained and upheld as icons of faith and identity, the respective chapters are too afraid to lose what little visual and material connection they have to their ancestral fathers, or it would simply be impractical to wield such weapons (I can imagine that Vulkan's hammer might be a tad too large and unwieldy even for the biggest of space marines). This vision of ancient archaeotech pistols, bolters, and eldritch powerweapons that cannot be used due to their inner mechanisms being beyond the grasp of even secret martian knowledge, and thus are instead polished and visually maintained as an anchor of faith for the sons of their bearers. A bolter wrought in the finest forges, by the most skilled hands and engraved with scenes and wisdoms of ages much greater than anything after the heresy, and yet it is decrepit, disfunctional, and broken inside; almost like the Imperium, in a way.
Or, y'know, perhaps another ad hominem attack on the design studio offers a better explanation.
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"What if the struggle between the Emperor and Chaos is a simplified example of Hegelian Dialectics that cannot reach a solution due to an insidious, capitalistic bid for the consumers money?" - A post by yours truly, 12th of August 2017, before the clock struck 1AM