Sure there were people who could have used them. But that's not the point. Sure, they were boots Fitzjames wasn't using. But Hickey's not immune to the optics of it all. These are the captain's boots. Boots that he had been buried in, boots that were buried in a way where they tried to hide Fitzjames's corpse from prying eyes.

Look at your captain now, huh Crozier? Look at where Fitzjames with his morals ended up. Dead, and Hickey's got his boots.

"My shoes were falling apart," said with a little shrug, as if that's answer enough (which in Hickey's mind, it is). "Waste not, want not, yeah? I'm not going to step around in boots that are falling apart when there's a perfectly serviceable pair I can use."