The lightsaber hilt of a Jedi/Sith has been cut by an enemy's lightsaber frequently during duels as most of you have already noticed. [url=http://www.youtube.com/watch?v=SOA9CpiWrI0&t=18s]Kazdan Paratus[/url]'s lightsaber hilt is made of a metal which is impervious to lightsaber damage. So why aren't other lightsaber hilts made of the same material? Imagine how different things could have been for Darth Maul when he battled Obi-Wan on Naboo.
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Because that metal is very hard to find. It's sorta like Star Wars' version of vibranium.
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It's a Mandalorian iron called Beskar, extremely hard to find but it is resistant ro any lightsaber attacks. Most commonly used by Sith instead of Jedi, since Mandalorians were never on the same side as the Republic.
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Beskar is a tricky metal to get ahold of. Any Jedi attempting to get it would probably be killed. As for Sith, Mandos don't trust them and they don't sell it to anyone.