Explaining The Eagles' commercial record chalking it up to casual listeners being the vast majority stops working as soon as you go down that list and find Led Zeppelin IV, The Black Album, The Dark Side of the Moon, etc.
Searching for other plausible explanations:
Quality hits, great hooks, three top notch singers in the same band.
The best band vocal harmonies since The Beatles
Conciling white America's favourite genres, country and rock'n'roll
Telling "don't even try to understand, take it easy" to an audience suffering from social turmoils/Vietnam fatigue
Writing, basically, the American answer to Stairway to Heaven
Disappearing while at the top, leaving fifteen years of nostalgia/discovery buys driven by Frey's and Henley's hits.
Interesting question: in your perfect world, what album should be the last century's best seller instead?