The gameplay improvement from AoE1 to AoE2 was Technology and Programming Creativity. Machines that could handle more computations went hand in hand with the programmers who implemented ideas that weren't yet possible with the earlier machines.
Nowadays, we got oversaturation of both fast processors and gameplay ideas. So much so that you don't know what to put in unless there are strict guiding principles of what kind of RTS you're trying to make. - One mistake is having all these ideas blended together that it made a Frankenstein's Monster of a game.
Also in today's corporate AAA industry, Game Companies no longer care what their game looks like from the inside (and even in this department W3: Reforged breaks that apart), as long as it "looks" great to play. The Open-Source world can't compete with this kind attention-grabbing competition. - That's another possible mistake: poor/false game advertisement