It's hard to understate just how important the postal service was to the country: beyond driving so much commerce (and hence making mail deliveries a valuable target) it was used for all kinds of official documents and financial documents (money orders were introduced to reduce the risk further). Travel being expensive and hard, no alternatives like phone or email, etc. means that you send letters a LOT and things like postmarks or registered mail signatures have a substantial legal weight. Making a highly trusted service with special status to deter thieves available to everyone at a modest fee was a really smart move.

Not merely threats to the postal service itself, but threats via the postal service. I imagine the best to get away with (say) fraud has always been to avoid physical presence, i.e. using mail.

I'm sure that the recent shooting of their FedEx colleagues in Indianapolis is on the USPS's mind as well, and I wouldn't be surprised if UPS and DHL tries to monitor social media as well.

also, back then, the USPS did more than just deliver mail.