2. Brock Lesnar is out of the picture

There’s a reason why Brock Lesnar was so heavily favored in his Royal Rumble title defense against Finn Balor. He’s a uniquely credible talent who has been protected like few stars in WWE history. It would have been odd—though not out of the question for WWE to have taken the title off of him that close to, but not at WrestleMania.
If Lesnar were to have dropped the title at the Royal Rumble, the logical conclusion to draw would have been that for whatever combination of reasons, Lesnar wasn’t working WrestleMania at all.
Indeed, the only other reasonable alternative might have been the Beast Incarnate in some sort of special attraction matches like a showdown with returning Roman Reigns or a carefully planned bout with MMA/UFC rival Daniel Cormier. More likely, though, Lesnar would be absent through at least WrestleMania.