WrestleMania is in the books and despite the show going 7+ hours, the celebrity appearances were few and far between. The long talked about match between Big Show and Shaquille O'Neal fell through, WWE's attempts to involve The Rock and Vin Diesel were thwarted by scheduling conflicts and Ronda Rousey declined an invitation to appear.
WWE had also courted UFC's Conor McGregor for over six months and while he was very interested in being involved with the event and the UFC was willing to cut a deal, it was actually former WrestleMania headliner Floyd Mayweather Jr. and others involved in the boxing industry that squashed the idea of Conor being a part of WrestleMania.
Conor is currently trying to work out a deal to face Floyd Mayweather in a boxing match in September on pay-per-view and Mayweather's team, as well as the PPV providers, did not want to take away from the uniqueness of the event by having Conor McGregor crossover to pro wrestling (even though he would have had a non-wrestling role) right before crossing over to boxing. The money WWE was offering was peanuts compared to what Conor will make fighting Floyd and while Conor didn't see much harm in doing something with WWE, it wasn't worth upsetting Mayweather and his team when they were adamant that Conor shouldn't be involved in any PPV events before facing Floyd later this year.
The Conor vs. Floyd match is far from a done deal, but it still played a role in WWE failing to secure a big celebrity for WrestleMania this year even though they went after several.