#3. It makes the most storyline sense
Seth Rollins vs Brock Lesnar may not excite many (mainly due to Lesnar), but the fact of the matter is that as of this standing, Seth Rollins vs Brock Lesnar makes the most sense from a storyline perspective.
Sure, Lesnar vs Lashley would be a dream match, Lesnar vs Cesaro is another one, but similar to AJ Styles and Ricochet being the right feud right now, the same applies to Seth Rollins and Brock Lesnar.
Roman Reigns in the picture would make absolutely no sense, even if he is the uncrowned Universal Champion. He is in the midst of another rivalry now and Seth Rollins has been the one who has held the Universal title for the past few months. It only makes sense that he would be the one vying to regain the title he lost to The Beast Incarnate Brock Lesnar.