The guy died in 1968. He slowly released the material among the course of 44 years; after his death, many of these documents remain unreleased. Kind of a weird behavior for somebody acting under the premise that "it was important for the world to read it".

He then left everything in hands of his secretary, who proceeded to, among other things:

* Sell plenty of Kafka's letters and postcards to private entities.

* Attempting to smuggle (out) some manuscripts without filing photocopies to the Archives of the National Library of Israel, which is required by law and is also, again, kind of a weird behavior for somebody acting under the premise that "it was important for the world to read it".

* Auctioned an original manuscript of The Trial, fetching about 2 million USD.

I couldn't find evidence of one single piece of material that was willingly donated to a museum or similar entity by either Max Brod or Esther Hoffe during the 83 years (!) they had it under their possession.

Even squinting very very very hard at the situation, it's hard for me to not see "the obvious economic reasons attached to it".

>But there are other things you relinquish by giving it to public domain

Like what?