This is why I dont really bother with those sets. While i assume they are best in slot the chances of getting them are so rare that it really isnt worth it.
They aren't rare, per se. It's just that RNG is... well, random. On one extreme, I have 18 Kena helms. And on the other, I have zero Bogdans. For the most part, I find that the helms have a fairly reasonable drop rate. But that doesn't mean you won't be unlucky with a particular one or a particular weight/trait combo that you seek.