#3 Increased revenue

WrestleMania is a huge weekend for everyone involved, and with the shared revenue from the PPV, the gate, merchandise, meet and greets, and so much more over the weekend, it brings with it so much success, but adding 2 shows instead of one increases that revenue further.
With Axxess, NXT Takeover, the Hall of Fame, and 2 WrestleMania shows, the WWE will make a killing over the weekend, and knowing how much the WWE seemingly cares about its fans, the money is likely the biggest motivator for the company.
Sure, they're likely not going to get 70,000 people to both shows, but if they can stick to running two shows of 40,000-50,000 fans and the product is better because of it, it seems a no-brainer.
Overall, it may not make a ridiculous difference to the WWE's earnings over this weekend, but if they can plan the few days better, bring in more money and keep the fans more involved, then that's what they should be looking to do.