#3. Because Lesnar is done with UFC
The Rock is a big Hollywood star and John Cena has followed him, pulled by the allure of the silver screen. Kurt Angle has retired from active competition and The Undertaker wrestles only on a few select occasions.
Like it or not, Brock Lesnar is by far the biggest star that WWE has at the moment, especially with Ronda Rousey on vacation at this time. And the fact that he has walked away from doing anything with UFC pretty much proves that he's committed to WWE, in the coming months.
Because Lesnar's schedule has cleared up, he can devote his time into making WWE his biggest commitment for the foreseeable future. Lesnar is a name that both casual audiences and hardcore ones know of, meaning that he is the company's biggest star at the moment.
Why wouldn't Vince McMahon want to shine the spotlight on him?