Instagram was a great product, Magazine a good follow-up.
It is marginally interesting that Magazine ownership is being transferred to Marco's partner, but I don't understand why that merits the #1 ranking on HN :)
Because enough people upvoted it to get it there. The Magazine, although not focussed on tech, I assume attracts a large tech audience (the people on HN). Marco is a well known person in the development community and I'm sure quite a few people saw the post on his blog and submitted here gaining it upvotes.
Instapaper was a little product made by this one guy at home that grew big and I think for that reason, Marco Arment's story appealed to a lot of people here that are hoping to build their own bootstrapped product. And now he's at a later stage where he used the Instapaper success as a stepping stone for more projects, like The Magazine.