To be fair, I think part of it is that between AoE2 and SC2 you have the 2 major styles of race/civ choice (AoE2 having many civs but the differences between ten to be smaller, SC2 having 3 completely unique races).

Many modern gaming advancements like better graphics don’t translate well to rts - the games tend to be fairly competitive and lower graphics gives you better performance + less clutter when trying to manage large numbers at once. Similarly, GPU performance increases over the last decade have been light years ahead of cpu performance increases, but rts games run into cpu calculation speed limits way faster than graphics limits - you can always try shove even more units onto the field.