#3 The FOX deal and making SmackDown look bigger at WrestleMania
It's hard to think that before 2019, SmackDown was never the flagship show of WWE and at WrestleMania.
RAW was always presented as the "A" brand in the company, but the big-money FOX deal has changed that.
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Slowly but surely, SmackDown transitioned into being a bigger show - in terms of viewership, money, as well as WWE's presentation of it. The blue brand is treated as a bigger deal and it has some of the bigger names on the roster as well.
This could be a token of appreciation to FOX Sports. Since the Universal Championship match from SmackDown is headlining Night Two, WWE would have wanted something similar for the first night as well.
As a result, there was only one actual match that made sense to main event WrestleMania other than the Universal Title bout.
There was a lot of dispute about which show was going to be the main event of Night One, but SmackDown wins this year.