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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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.