"It amazes me that museums will sell off their blades, but I've seen it done several times, here in Hawaii, in Arizona, & in Wyoming. I wonder if these sales may be related to today's knee-jerk reaction to weapons, in general. I asked the curator of our local museum, but he refused to answer me, which I guess is an answer, in itself".
So, Ken -- would you really like to know the answer to you question? Well, to put it simply, it's because museums are not run by men who might/would see the value of swords, armor etc. but by 'very' rich women, who sit on the boards of said museums and create the funding for said museums - who do not. If you doubt me, look at any exhibit being funded by a married couple and you will see the wife's name first. The Met can get away with a sword/armor exhibit because they have a long standing armor and arms club which is the wives way of placating 'hubby'. All of this has nothing to do with cultural, historic or artistic preservation and only to do with their own 'social standing' and hierarchy. Trust me, they care not a whit about Nihonto but will swoon at the sight of some piece of trash which is only important because their lap dog, sycophant dealers insssist they are (yes, with three s'sss), and of course that will increase their position forward at the table! Now then, this is not to say that all women feel this way and there are some who truly appreciate these fields and we all love and appreciate them for it but in the greater picture, they are a small minority.
So, the next time you see a museum deaccession their Nihonto or other armor related items, you know where the funding came from for the latest 'acquisition'.