She tried. When she told the other Diamonds she didn’t want to go through with the Colony, they told her to finish what she started. When she told the other Diamonds she wanted to preserve life on Earth, they created the Zoo and threw a handful of Humans in. She did everything she could as Pink Diamond; but her status meant nothing to Blue and Yellow.
Okay but imagine if YD and BD give Steven the Earth, right, because he has PD’s gemstone, which basically makes him a ruling figure.
Well, a ruling figure needs a court, right? And Steven’s like no way jose I got my fam right here, but they still send him like a Starter Pack Gem Court of like a Sapphire to predict danger and maybe a Zircon to uphold justice for him- you know, the basics- to try and get him in the Diamond mood.
And these poor Gems are like “we’re about the serve the newly found PD, what the f u d g e” and are worried out of their wits because they dunno what to expect, and when they get there they find some little squirt wearing sandals. Thoroughly confused, they’re like “what do you desire my Diamond???” and Steven hmms, taps his chin, and is like. “I dunno. What do YOU wanna do today???”
Basically what I’m getting at is that YD and BD trying to make Steven into an Official DiamondTM would be utter chaos for them because suddenly all of Homeworld hears about the new, chill Diamond who just wants to hang out on a beach and play music and suddenly there’s, like, a mass migration of Gems to Earth to strum ukuleles and eat popsicles and the Diamonds suddenly suffer a huge worker class collapse because they’re all tired and want a fucking vacation.