jamie The game is super complicated and experimental; balance is going to take TIME. Of course they are doing good stuff to make it better, but that does not mean it is actually balanced. It took Overwatch 6 years to even start to be "balanced" and it still goes out of whack every other character release.
I think system requirements play a larger role than most dedicated gamers will think. Overwatch can run pretty good on as low as i5, 1000 series gpus. Marvel rivals is not even ideal running on i7, 2000 series gpus, certainly not with other applications running. This is going to severely limit PC players, who would use their family computer, school laptop, etc. What numbers we don't have here are consol player numbers, and I would assume THIS is where marvel rivals crushes overwatch. The more forgiving mechanics are more satisfying on controller, the Marvel IP has higher impact on casual gamers, and for players used to buying COD, Madden, 2k, Fifa 999 etc. The newer game is going to seem an obvious choice. All of that in smaller ways can apply to PC as well which is why I believe the system requirement difference is likely a massive contributor to the surprisingly close steam numbers.