If Langford's team could reach Heliopolis by stargate in 1945, why was Abydos the only planet SGC could dial before discovering the Abydos Cartouche?

In SG1 Children of Gods (episode 1.01/02), Samantha Carter concludes that, without accounting for stellar drift using a map such as the Abydos Cartouche, the only planet reachable by Stargate is Abydos because it is in some sense the most local relative to Earth.

In 1945, Paul Langford, Ernest Littlefield and their team successfully dialled Heliopolis, (SG1 The Torment of Tantalus, episode 1.11) implying that Heliopolis, too, is local enough for the Earth gate to reach it without adjustments in the 20th century.

Why then was Heliopolis not found when SGC were trying to find alternative destinations to Abydos in the aftermath of Stargate (the film)?