Ever heard of FSO?
Yes I have, and although I have never taken part in one, I am reasonably certain that if the He-111 were to be added, the CMs would not change every FSO to include He-111s. Sure, that plane would then be able to participate in them, but it would not be in every single one. In addition to this, your average Aces High player would likely be far happier to fly in a 300+ mph late war bomber, than in a ~200mph early/mid bomber that had a maximum bombload less than that of the Ju-87.
I would think that when looking at the current plane set, in terms of substituting aircraft, it is far easier to substitute a Ju-88 for an He-111 that it would be to substitute something for an A-26.