#1 It's been a long time since Mysterio held a World Title
In the past few months, Rey Mysterio has unsuccessfully challenged for the Universal Championship many times. Now, he's going to do the same with WWE Championship, when WWE makes this official. His last World Title reign came back in 2011, when he beat The Miz for the then-vacant WWE Championship, but he lost the title two hours later to John Cena.
Since then, The Master of 619 hasn't won a World Title in WWE. He's on the verge to retire from wrestling, and he teased it some time ago on RAW, and it'll be unfair if WWE doesn't allow him to have another World Title reign.
Undoubtedly, beating Brock Lesnar is a huge feat, but Mysterio has proved time and time again that he is capable of picking up big wins. He has always worked as an underdog and this is what will make his next World Championship win special if he manages to successfully challenge Lesnar for the title.