If you don't mind a few "legends" or essentially non-canon answers, here's an explanation:
Due to the weightlessness of plasma and the strong gyroscopic effect generated by it, lightsabers required a great deal of strength and dexterity to wield, and it was extremely difficult—and dangerous—for the untrained to attempt using.
Lightsaber combat was difficult to master for a number of reasons, one of them being that all of the weight a lightsaber had was in its hilt, and the gyroscopic effect caused resistance to changes in motion, or built up momentum so quickly than an untrained wielder could lose control of the weapon.
Basically, the point here is that lightsaber combat could be very difficult due to the fact that all the weight is in the hilt of the lightsaber, not the blade. Hence, making the lightsaber out of Bes'kar would make it incredibly heavy and unwieldy. Thus, combat could be a bit more difficult.
Also, to parrot another answer, Mandalorians and Jedi have a rather bad history, so very few would ever make that exception to work with them.