No reason it shouldn't be math instead of rex.
Well there are a lot of reasons whether you want to admit it or not.... I'd just rather see Math the battler rapper right now than Rex the battle rapper. Unfortunately this is a business deal, and y'all heard what Tay Roc said to Math in his freestyle "Y'all ain't selling tickets, that's why Smack, ain't booking nyggas"
Rex is a bigger name than Math.
Rex brought a small card headliner battle over to URL for free (vs Danny Myers), so they locked him in for SM5 back then.
Meanwhile Math lost money for URL his last time up there, really if you pay attention he doesn't do anything to help anyone get a leg up besides himself.
Math tried to get the Tsu Surf battle going, but struck out. He wants a big name, he might not have even been willing to take Top after Top lost to Suge. He was open to taking Goodz just to get on the card, but the fan demand for that battle isn't there. He tried to bait Ice after he knew the Surf battle wasn't going to happen, but it looks like it's Ice v Suge.
Math is just stuck on getting his way, and will undercut damn near anyone and anything to get it. He could just wash up one of these young guys THEN take a big battle on URL, but it doesn't seem like he's willing to do that.