Could they be more diverse? Yes. Would that be better? Yes.
Is it ok that they're not diverse? Yes.
Is it silly that they look tremendously similar? Pretty much, yeah.
Would people be making jokes about the unlikeliness of a diverse group of Assassins running and murdering freely around Paris during the revolution if there were a woman and a Haitian on the cover? Of course they would.
People would be complaining about this any which way. Regardless of however international the Assassins are supposed to be at this juncture, the fact remains that at that time period women were subjugated and racism was widespread, even among subgroups of West-European Caucasian. As much a fan as I am of diversity when used well, it's perfectly fine not have diversity if it doesn't add to plausibility, moreso if it complicates things. For instance, while women and Haitians and whatnot did participate in the revolution, that does NOT mean that their status as a member of another gender/race went unnoticed or unremarked, and it would have lead to complications that Ubisoft probably wouldn't want to bother with, particularly given the co-op angle.
Seriously, if they did have Joan of Arc and Alexandre Dumas running around, and the characters didn't react to it, people would complain it was unrealistic/dismissing real issues of sexism/racism. If they included them AND addressed those issues to a realistic extent, people would complain the game had bloated writing and spent too much time saying "Look at us, we have a female action hero in historical France, ISN'T THAT WEIRD?". People will whine and complain regardless of the choice made, so I can't blame Ubisoft for deciding to just make the game the way they wanted to.
Although, you never know, talk about it enough, maybe they might decide to throw in some last minute changes. Personally, I would like to have some diversity, but my point is I don't blame Ubisoft for not taking that route.