The spear and shurikens were acid proof, the rest was not.
I guess it was anderson's idea of balancing. maybe also having something to do with making the hunt more challenging, as well as perhaps the idea as to keep the aliens at more a distance, thus using the longer spear and the throwable shurikens more, and the only using the wristblades in a jam. You know, so the kiddies arent encouraged to go after something out of their league (they are young after all) or something.