@xs4me2
Not the least because it was law to own one, or something along those lines.
@benroyce

@alterelefant @xs4me2 @benroyce There was strong social pressure at times, especially for those connected with the State apparatus. And there was some overt public pressure from e.g. speakers and writers, that owning a copy was the duty of every patriotic German etc etc. It did no-one any harm to have a copy prominently displayed in the home. For a while, newlyweds were offered a copy. There was even a special wedding edition for the purpose. Some sources say it was "offered", some say it was given, some say couples had to pay for it. Maybe it depended on the location or the official presiding. Regardless, it would have been a bit tricky to say "no". Some groups, like the SS, some teachers, journalists etc were required to read part or all of it. The book was a popular school prize (popular to give, I guess probably not to receive). But I'm pretty sure there was never an actual legal requirement to buy a copy.

@kauer
Due to the extreme societal peer pressure there was no need to put it in law. The result was however exactly the same. To not own a copy or to decline a copy when it was offered by someone would bring one in massive trouble.
@xs4me2 @benroyce

@alterelefant @xs4me2 @benroyce From my reading, societal pressure was there, but it was largely related to official contexts. Most Germans at the time did not own a copy. The population was about 80 million, and only an estimated 12 million copies were printed.