The 70 pound limit is for all USPS packages. It would be possible to exceed that limit for the medium or large flat rate box, or a custom box.

The weight limit has been raised and lowered over time. It was first 70 pounds in 1913 for nearby addresses and 70 pounds anywhere domestic in 1931. It was lowered in 1952, but has been 70 pounds since 1983. Source: https://about.usps.com/who/profile/history/universal-service...

That doesn’t exactly answer why. Based on that page, the answer is the same as for most things like this: a complex mix of government, commercial and logistical factors decided by committee. There doesn’t appear to be a “fun fact” answer like “most postal workers after WW2 were infantry veterans and the limit was set to be the same as a crate of ammunition”.

I believe it is connected to the requirement that mail carriers regularly lift things of 70 pounds maximum weight.

A parcel heavier would therefore be a two-person lift and not conducive to the lone mail carrier model generally used for USPS last mile.

Looks like that’s the max weight for any package.

Possibly it's the calculated limit of some type of package-handling machinery. Maybe it's more interesting, like the original planned limit was 7kg and somebody in the editorial chain mistakenly thought the conversion rate was 10lbs/kg.