Here is my answer, based on many shiurim and ma'amarim from Chabad, who explain it. I've pieced it together over a lot of sources and shiurim, so it might be a slightly flawed or incomplete answer, and I also don't have anything particular to share as a source.
There are two skills we need to develop to fulfil Hashem's plan of Oneness (becoming One with Him and each-other); the ability to fight darkness, and the ability to appreciate and increase light.
The former is simple enough (although certainly not easy). One can achieve this with fasting (King David), or suffering (Mitzraim, galus), or an amazing event of emuna (Har Sinai). All of these things produce in us this uncanny ability to be able to deny the self. This is what it means to fight darkness - I can get myself out of the way and do a mitzva properly...
Our ancestors, the "mighty ones", were at the pinnacle of strength in fighting darkness due to all that they went through, as well as their own internal strengths and efforts. This is something, however, that erodes over time as we get more distant from these events and people (although Hashem's plan must succeed, so we go through a lot of suffering to keep us "topped up" with this strength). This is Yeridat Hadorot.
However, the other ability is much more subtle and can't be "injected" into us or "conquered" through great strength. The ability to appreciate light, life, goodness, value in others and Hashem takes thousands of years of study, sincerity, effort and working on ourselves. I'm a bit unclear on what we can say regarding specific ancestors and tzaddikim of the past, but in general, the human condition started off with very little of this ability. We used to keep slaves and do all sorts of things that people nowadays find highly distasteful and bizarre - we can't relate to it. This is because by following Hashem's Torah and being diligent and hard work over many centuries and millenia, we have brought this Godly ability to appreciate goodness into the world, into the human condition, and we now appreciate the supreme ineffable value and important of the "someone". We abolished slavery recently as a good example of that. We hear of stories of e.g. how the Taj Mahal, once built saw all its builders hands cut off because "no building should ever be built like this again" as not "deep" and "cool" but horrific, disgusting and nonsensical.
This is Aliyat Hadorot.