He COULD have, Should have implies he had all the skills, and while on the surface he has Vicious KO Power, and good wrestling to boot, there's more to winning than just fighting. When it comes to the Fighting aspect I don't think anyone denies that the dude had Championship qualities, but when we look into his Mental...that's where it goes down hill.
Physical Attributes are good and all, but the Fight Game is still mostly a mental game, and that's unfortunately where Rumble fell short. Having all the tools mean nothing, if you don't have the right mental to use them. Whenever the going got tough, he folded, gave up his back, and got RNC'ed(Yes, I know, like Conor)
This is gonna sound harsh but he's mentally weak minded, and he's very open about being disappointed in himself for not living up to his Potential. In an AOF Video about Usman winning the belt, you could see it in his eyes when he talks about it. He knows deep down he fucked up.