Because it was required that a copy be registered with the Library of Congress back then, if you wanted to copyright something.

edit: not sure why the downvote. I'll expand a bit.

The Copyright Act of 1870 required two copies of a work to be deposited with the Library of Congress in order to register the copyright -- creative works were not automatically under copyright.

Once that work passed into the public domain, anyone could request a copy of it from the Library of Congress (and then, if they wanted to, copy and distribute themselves). This is pretty important, because it pretty much guaranteed that people would have access to a work even after its copyright expired and was no longer actively being printed by the publisher or author.

You're probably getting downvotes because you're still required to do that to register copyrights, which any remotely commercially-significant work will do. (You can only sue for damages in copyright infringement cases if the copyright is registered.)