I was not fully recognized how much the altitude of Mexico City aided athletes (more recently, Allyson Felix ran 22.11 as a Junior and her best in the lowlands was 22.51.
As for Beamon, not only did he have the altitude, he also had a lot of wind. It was listed as 2.0, but there is A LOT of evidence that the wind readings were done incorrectly when person B was doing the readings. All of person B's readings were integers, like 2, 3, and some 1s. When person A was not doing the (preliminary?) rounds of the hurdles the readings were done correctly and you get the dominant proportion (90%) with some tenths recorded. The readings between 2.0 and 2.9 were all given a reading of 2.0, so it is 90% likely that it was wind aided. See the thread on the Track and Field News discussion board back some years ago (4-8?), including commentary from people who were there. I remember watching on TV as a teenager.
There were no more stellar long jumps that day because right after Beamon's jump it poured very heavily. I think that in the triple jump at the 68 Games, the World Record was broken at least 3 times. Also, the warm, humid air right before the storm hit would have also helped a little, and if I recall correctly, he got all of the board. Absolutely everything was right for that jump: (excess) wind, perfect board, very high altitude, warm/humid air (but unless it was hot it does not help that much. But if there was a heavy storm, it was likely a low pressure system which lowered the air density and helped also.
I think that Beamon set the world indoor record later, but I am not sure that he ever jumped 28 feet. In part, he could not reproduce jumps in that range.