After spending ten days to prove or confute theories I have only partial results, so here are my conclusions:
Theory #1: Oxidation
Confuted (imho). These confuting experiences was included in my original question but for have it in an organized answer I list here:
I had a one and half year old, accidentally opened (originally vacuum packed) D'Addario Chromes (flatwound), and I stringed my guitar with them ten days ago. The strings sounded the most typical new string like. (still no significant changes after ten days). So its not the oxidation, because the opened and same age strings were oxidated almost identically, but sounded way different.
Theory #2: Visco elastic dirt
Confuted in two ways.
a) Reverse confuting: I've tried to clean (I mean deep cleeaan) my old strings. (See details of the process in my comment on user2790167's answer.). No hearable changes at all. Maybe the clean was not "deep" enough, as user2790167 answered: we did not reach the core... Still I would expect some sound changes, but there was not any.
b) I've tried to artificially speed up aging the new strings by dirting them with a chapstick. It is kinda gluey fat so it's a candidate replacing nose grease with the benefit it is available in limitless amount... (nose grease credits goes to user2790167)
I even extra strengthen the strings during the process to dilate the ways to the "core". No changes. I thought it takes some time while the dirty travels to the core so waited five days before concluding the result in this answer: Still no changes.
Theory #3: My own
Note: This is not proven at all. This is only a theory what based on that neither Theory #1 and Theory #2 seems to be not provable. (although both sound reasonable).
So... That's a fact, that new strings sound are well noticable changing in the first 1-3 month after applying them. If it is not the oxidation and not the dirt, then what else? The only thing what comes in my mind is the "usage". The steel's attribute must change somehow, my bet is its flexibility. This could be because of the micro dilatation, or some microscopic changes in the matter itself. The steel gets tired and tired.