It might be less of "who is more deserving" and more "who has waited for years and might not get in if I don't push him in this time and who can I not vote for because they are at no risk of not eventually getting in".
It's like the baseball HOF. Only one guy has ever been unanimous because even when there is a no brainer, it becomes about people starting to use their votes to help at risk guys.
For instance, Belichick's far more deserving than Roger Craig. But Craig's been eligible since 1998 and has been waiting and we are going to approach a point where if he doesn't get in soon, people who didn't watch him are going to start becoming voters and he might never get in. Another scenario is that Kraft getting passed over for so many years meant people might have been unwilling to give two slots of the 3 to people from the Patriots. So by passing over him for so long some people who wanted Kraft might have made a decision that he was going to be the Patriots vote for this cycle. Then there's people like Ken Anderson who waited 30 years.