As Hartley said "The past is a foreign country; they do things differently there."
This period of Democratic dominance in the house was during a major shift in American politics, and different parts of the Political system shifted at different times.
In the 70s and 80s, the rural South was Principally Democrat in Congressional elections and Republican in Presidential Elections. This, combined with Democrat strength in the cities, resulted in a significant advantage for the Democrats.
There seems to have been a very different view taken by rural southern voters of local Democrats standing for House elections, and National figures, standing for President, and they may have been right: Representatives like Jamie Whitten of Mississipi were from a different era of politics and socially conservative, compared to Walter Mondale (a Northerer and liberal).
The incumbency effect is strong in America, and the two landslides were elections of popular incumbent presidents. But incumbency also preserved Democrat seats in the south, even among voters who voted Republican in the Presidential race.
Finally this was a period in which many chose to use "ballot splitting" believing that it was better not to have one party in control of both Congress and the Presidency. There were voters who typically voted against their presidential pick in the House or Senate races, in order to preserve "balance".
So the past is indeed a foreign country. Now ballot splitting is rare, and the rural Southern voters now mostly split along racial lines, giving reliable Republican votes in both presidential and congressional elections.