Because the idea is for the entire generation, not the two halves of the generation.
Except the shift from XY to ORAS was equivalent to prior generation changes (think DPP -> BW), not mid-generation changes (BW -> BW2) in the sense it completely changed how ubers is played. Outside of the big 4 that dominated both metagames (Xerneas, Gengar, Arceus-Normal, Ho-oh), there was a massive shift in what was considered viable. By the numbers, 40% of S rank, 33% of A+ rank, 14% of A rank, and 56% of A- rank (37% of pokemon in ranks S to A-) experienced a major drop of 4 subranks or more in the shift from XY to ORAS. Moving past this, the core of pokemon that remained in the A ranks through the shift no longer run the same sets from the XY era in ORAS ie bulky SR Lando-T being the only XY set vs Scarf the only ORAS set, SpD/LO Dialga being the only XY sets vs offensive Shuca being the only ORAS set, WoW fast support Waterceus vs Toxic Bold Waterceus, etc, etc. Basically, there's such a large difference between what mons are viable and the roles they provide that you can't group both metagames together and make accurate generalizations about them.
I did say quite clearly in the OP that this was for both XY and ORAS, and separating them when, as you said, many people haven't played XY in a long while would just lower interest in the projects, and also diminish good conversation.
I feel like this point is slightly contradictory; if people haven't played XY and can't give educated opinions about the metagame, why include it (besides the obligatory "because gen 6 metagame" response which I addressed above)? It's obvious that the majority of nominations were made with only ORAS in mind, as dominant XY mons (read: S ranks Kyogre and Mewtwo) and those ranked highly in both metas (Dialga, Yveltal, Deoxys-S, Giratina-o) were left off the list. Plus, a decent chunk of the voters, myself included, voted strictly off ORAS viability (just read the reasoning for everyone but Hack and Klefkwi).
Sorry if this is derailing the thread but I needed a reason to procrastinate.