They aren't. It says right on the page that only 20% are actually online. Additionally the site is now archived and doesn't get any further updates.

A shame really that an university department needs/needed to rely on external funding to digitize the maps it already has and apparently they didn't get enough money and now stopped the effort

Or maybe they didn't stop the effort and only this page is not the "go-to" place anymore. Bit unclear to me