It was not a mistake #1: Wrestlemania 35 is already an extremely long card, and eliminating a match will cut the show's run time
One problem that Wrestlemania sometimes encounters is wresting fan fatigue. Most movies, plays, and other forms of entertainment usually don't run more than two hours because that is how long most people can sit comfortably.
This is why Avengers: Endgame may have an intermission during its three-hour run time available. With a stacked card chock full of emotion, fans might be fatigued after the projected four-hour run time, and not be able to appreciate the main event.
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Cutting out the Smackdown women's title match will shorten the overall event by, on the average, fifteen to twenty minutes. This gets the event closer to the threshold of comfort for both the fans in attendance and those watching at home on the WWE Network.