Probably because she wasn’t given the same reverence as her mother so she was allowed to
have real flaws.
Manga Usagi is actually much more realistically flawed than 90’s anime Usagi though. Her jealousy towards Chibi-Usa is taken as an actually serious and dangerous, thing, she’s forced to acually confront these ugly feelings inside of her during the Black Moon Arc and grow as a person as a result of them. In the beginning of the manga her cowardice and spoiled nature as a Princess is outright deconstructed as she selfishly repeats the same mistake that she did in her past life - commits suicide when all looks lost and nearly wasted the precious second chance at life Queen Serenity offered her and the Senshi.
Not to mention that even at the very end she has the realization to admit to herself that she was never actually fighting for “pure” reasons like love and justice like she convinced herself, but rather simply to selfishly protect HER people and preserve the peaceful life she’s always wanted for them. This is why Manga Usagi was often needlessly cruel and merciless towards her foes compared to her anime counterpart, she was too short-sighted, selfish and immature to even offer her enemies the chance at redemption because she was always only thinking about how it’ll affect HER life first and foremost and simplistically dividing good and evil into this rigid Black & White binary with which she needs to cleanse the world from in order to establish her perfect utopia rather than accept Chaos into her heart and the lives of everyone - because she eventually realizes that Chaos is ultimately just the same as her, a lonely star seeking companionship and completion.
Chaos is not the enemy, it is simply a fact of life, and she spent too much time trying to run away from her problems and denying it’s place in the world rather than accepting and embracing it. (We even get a glimpse at an incredibly flawed/morally grey far future version of Usagi in the form of Cosmos who had become as Machiavellian Ruthless and Jaded as the 90’s anime Outers themselves and was willing to risk throwing the existence of the entire universe at stake just for a chance to end the existence of suffering and evil itself - this is who Manga Usagi would’ve eventually turned into if she hadn’t had that important realization at the Cauldron by staring Galaxia straight in the face and seeing herself in her.)
90’s anime Usagi by contrast is much more so “superficially flawed.” As in, her flaws are never presented as actual flaws that hurt her own moral character but are more like Moe quirks that are just there to make her look cute. While Manga Usagi is more of a “Status Sue” (i.e. the type of Mary Sue whom the whole universe revolves around) 90’s anime Usagi is a straight up Purity Sue and is always characterized as the ever picture perfect paragon of pacifistic and Jesus-like forgiveness. She is never allowed for her views to actually be challenged or to display any real darkness in her heart, not really. While Manga Usagi had to eventually grow into the realization that fighting was never the answer by confronting the ugly side inside of her, 90’s anime Usagi is endowed with this mindset from the very beginning and is proven right over and over again as she even manages to displace the
actual most powerful/important Senshi of the Galaxy by becoming literal Jesus of our entire Solar System itself.