#1 Will keep fans invested in every match

The WWE has enough trouble getting its fans invested in all of their stories and matches heading into WrestleMania, because the storytelling isn't what it used to be, but asking them to sit through 16 matches makes it that much harder.
The usual PPV run time is between 3 and 4 hours, but WrestleMania has bloated out to 7-8 with a pre-show, and a 2-day event allows them to skip the pre-show, and just have fans get invested in two quality PPV events over one weekend.
This year, the WWE has built up 3-4 matches to be must-see over WrestleMania weekend, with many others just thrown together, and if the WWE looks to make 'Mania a two-day event, they can spend months building a plethora of important matches, making it a much more enjoyable event for fans, worthy of the name - 'The Showcase of Immortals'.