#3 Leaves fans satisfied
Let's do a doomsday scenario for a moment and imagine that WWE is forced to shut down for a period of time after WrestleMania 36 takes place. While it would be devastating for the WWE Universe and probably force the company to air reruns for several months, WrestleMania 36 would at least be in the books.
Not only does this allow the company more time to plan what is next without the rigors of weekly television, it also at least gives the fans a satisfying conclusion for the time being. At least they saw Drew McIntyre vs Brock Lesnar, or Roman Reigns vs Goldberg, and were given the payoffs they deserved for investing in the storyline.
In all honesty, to leave things unfinished would be an absolute travesty and putting it off could ruin the magic of WrestleMania entirely. That's why having WrestleMania being a two-night taped event at least allows the company's current storylines to end, gives them the chance to create new ones during the off period and leave fans wanting more!