#4 He is a fresh challenger
Brock Lesnar has had a number of high-profile feuds since his return to the WWE. Being the big draw that he is, it is inevitable that The Beast Incarnate will only be confined to these big matches rather than mid-card affairs.
He has faced off against the biggest names in the company including the likes of John Cena, Triple H, CM Punk, The Undertaker, Bray Wyatt, Seth Rollins, and Roman Reigns. Those last three names are specifically important for this point.
Female superstar says she inspired John Cena turning heel HERE
They made up three out of the five participants in the Extreme Rules Fatal Five-way match. A victory for one of them would have just been a repeat of a feud that has already happened. Instead, we need a fresh challenger to step up against Lesnar, and that fresh challenger is Samoa Joe.
Why not Finn Balor, you ask? That brings me nicely to my next point...