IMO because Leonard's mind games, Hagler became so obsessed with proving the world he was the superior fighter than he became a victim of his own hubris. Kinda Marvin’s hare to Leonard’s turtle. I mean, the general opinion was that Leonard was the smart and fine stylist while Marvin was somehow unjustly portrayed like just a slugger with a great punch and a great chin, but technically inferior. Maybe that finally got under Marvin’s skin. Thus, Marvin came to the ring not just aiming to beat Leonard. Marvin wanted to prove he was the superior fighter by defeating Leonard in an absolute and commanding fashion. Marvin wanted to accomplish the perfect fight … but as we know, sometimes “perfect” is enemy of “good enough”. Maybe that was the reason Marvin started slow and wasted rounds. Compare what happened when Marvin fought Tommy Hearns. Marvin knew he had to take Tommy down the sooner the better and from round one he was firing in all cylinders. But versus Leonard Marvin was in no hurry. He was taking his time setting his perfect fight. Marvin was just savoring the moment and didn’t want to rush things. He didn’t want to scare or ruin Leonard too early. That would spoil the boxing exhibition Marvin intended. Thus, let Leonard look good for a while and take some advantage. That would make Marvin’s victory all the most impressive. Marvin was feeling so high and sure of his victory, since he was in his mind the undisputable better fighter, that he didn’t realized he was actually playing right into Leonard’s hands by underestimating him, providing Leonard with the time and space to start raking points and stealing rounds with his “looking good” tricks. When Marvin realized it was a real fight what he had in his hands and tried to hurry things it was already late. Just like the hare.
Maybe hubris ... Maybe Marvin was so eager to grant Leonard all the available advantages because he didn't want to give Leonard and his apologists any chance to justify or excuse his defeat to Hagler.
Not directed at you, but I feel compelled from time to time when I see this to remind of the facts: This fight took place in 1987. Marvin hadn’t fought a scheduled 15-rounder since 1984, against Hamsho the second time, a full 2 1/2 years earlier. He had long moved on from that and 15-rounders were being phased out. In negotiations, Leonard conceded money (Hagler got by far the bigger purse, $20M to $12M iirc) and Marvin traded logistics like ring size and glove size (middleweights had long been either 10 or 8 ounces rather than set size) for filling his bank account.
The fight was close but Hagler underperformed, fought dumb and Leonard stole the event. Marvin has no one to blame but himself. In exchange for the larger purse he gave away ring size, glove weight then boxed instead of tearing into Leonard allowing him to get set and gain confidence, fought him orthodox, a total poorly played hand. Plus he was long in the tooth.. Ray picked the perfect time to fight Marvin who while still super tough had slowed down enough to be vulnerable to speed.
Not disputing that but the way the fight was going it was possible that those extra rounds would have paid off for him. It was one of Leonard's conditions for the fight alongside glove size and ring size. Not saying that's the only reason the fight went as it did, we will never known but I do think it was a mistake.
I thought Leonard edged it and Hagler threw away the first few rounds because he thought Leonard was going to basically collapse after 6 rounds. I’ve never bought into the whole “you have to take the belt from champion” thing, and in an interview with Sky Sports a few years later, Hagler wasn’t pushing that line anymore either. He said he’d thought that’s how judges scored fights, thus explaining why he didn’t get the nod over Vito. When it then didn’t happen in his favor with Leonard, he figured it was just because the judges didn’t like him (!).