When going to two you need to handle split brain some way probably, otherwise you end up with an database state hard to merge, thus you better get three, so two can find consensus, or at least an external arbitration node, deciding on who is up. At that point you have lots of complexity ... while for HN being down for a bit isn't much of a (business) loss. For other sites that maths probably is different. (I assume they keep off-site backups and could recover from there fairly quickly)

There are plenty of sites where it's acceptable to be down for a bit sometimes.

Having two servers is a lot more than 2x as complicated and expensive as having 1 server.