[ig1]

Ignoring the fact public domain gets complicated when you're an international organization (there's many journals which are public domain in the US but aren't anywhere in the world because of historical anomalies in US copyright law), JSTOR is essentially built on relationships.

JSTOR has access to the current archives of the journals because of the relationships it has with the publishers, if the publishers weren't happy with what JSTOR were doing they could just pull their licences for their current journals. If that happened JSTOR would basically collapse.

It just doesn't make sense for JSTOR to do it, when a third-party who doesn't have any relationships to the publishers could do it just as well (you can easily get physical copies of most historical public domain journals either via the open market or via libraries and scan them).