Probably for the same reason that the Internet Archive decided to thoroughly archive all of GeoCities when that closed. People tend to like preserving what they view as being important history.

Also, not wanting a billion bookmarks to vanish is a good reason. Plus I'm sure he can convert a bunch of former Delicious users over to Pinboard simply by operating the service.

Considering it was sold for $35k the value is not high in monetary terms, but there are other kinds of value.

It sounds like from the post he's not thinking about it in monetary value.