The bookies were in favor of Joshua because people were more familiar with him and that greatly influenced on the odds. The bookies made odds to look like a Joshua is a clearly favorite in the fight. But in reality all the odds must be identical.
I've read and watched dozen of boxing specialists reviews. All of them were saying there is no clear underdog and a favorite, and all of them added that Usyk is a very, very inconvenient opponent.
I am even sure, people who made bet, are not even familiar who Usyk is. AJ - huge, heavy, knocked out Klitchko, has titles. People are familiar with boxers from kings weight - heavyweight division. Who is that cruiserweight from Ukrain that want to face champ? No idea. That what majority thought. If we ask people who is champion in cruiser or light heavy, people will just shrug.
The bookies are not really in favor of anyone except themselves when it comes to giving the odds to any sport event, what happened is that the people forced those high odds.
It is similar to what we see in this market when we see FOMO taking place, a coin which does not deserve it goes up in value because the expectation has been created that it will keep going up in value, in this case many casual fans did not knew who Usyk was so when they had to make their bets they decided to bet on Joshua simply because he was the favorite.