The Watchtower organization was much smaller then, so the initial print run was small, also, that was long ago, and lots of dogs have eaten a lot of homework in that time, also, no doubt many copies were destroyed when the US gov't cracked down on the Watchtower for violating the Espionage Act.
Today a heavily-revised version of the book is kept in print by people who think that Pastor Russell was the man with all the answers - never mind that NOTHING Russell predicted came to pass - unless you count INVISIBLY, in which case it ALL CAME TRUE.
As pointed out, if you simply wanted to satisfy your curiosity, you can do so for free. If you are jealous of antiquarian booksellers making a killing, work on improving your character. Envy is ugly and you are better than that. (Yes, I don't know you from Adam, but envy is ugly on EVERYONE, and EVERYONE is better than that.)