Because Sylvia was so bloated, out of shape, concussed and injured that multiple boxing and MMA commissions wouldn't even sanction him to fight. The fight was finally scheduled as a boxing match in New Jersey, who refused to sanction it under MMA or Boxing right before the event, so they moved it to Alabama a week later because of Alabama's incredibly lenient sanctioning rules as a Boxing match, and even despite their lenient rules they decided the day before the event that they STILL wouldn't sanction Sylvia for boxing, only under MMA rules on the basis of Tim's previous MMA accolades despite his failing health. They made a gentleman's agreement to box with MMA gloves, and you can even see Mercer give him a look when Sylvia throws that first leg kick and Sylvia looks at him like "my bad, I forgot" and then gets KTFOed.
It's just not a legitimate comparison when you consider how fucked up Sylvia was at the time. He wasn't even allowed to train by commission ruling for 180 days after fighting Fedor, then further injuries after stopped him from training at all against Mercer. He openly admitted that he didn't even care about the fight, and just took it for money, saying that he figured it was free money, and losing to Mercer in boxing wouldn't hurt his MMA reputation. He just didn't give a fuck and showed up when they told him to. So not only was he so injured and out of shape to the point that most commissions in the country wouldn't sanction him to fight, he didn't even attempt to train.