As their dating life started getting more serious, Moonstone wanted to. But Carlosruizite always had something very scientific to work on, or promised to show up but couldn't make it, and they didn't get around to it.
Moonstone tried to talk himself out of worrying about it, to cast it as a healthy and positive thing. "We are not one person. How lonely that would be! A couple who has made themselves one so completely, that they are once again alone."
Even though, for Gems, fusion isn't unhealthy or loneliness-inducing. A healthy one involves connection, communication, and togetherness. (Which parallels how, for humans, "telling your partner basic facts about your career situation" is healthy communication. What's not so healthy is telling them "you need to put up with the consistent unhappiness of this LDR because it's a good thing for my career...and you might start second-guessing this logic if you knew I haven't actually talked to my employers for years, but, conveniently, I've never brought that up.")
Then Moonstone got imprisoned by Strexcorp, and Carlosruizite went MIA. Even when Moonstone was rescued, and spent a little time hiding out in the otherworld desert with the Giant Army, Carlosruizite hadn't made it there yet. Moonstone never asked what he was doing, or why it took him so long.
The events of the next year combined to push Moonstone into a deep depression. His gem projection was poofed several times; every regeneration got sloppier and less coordinated as he put less effort into it. The more adrift and disconnected he felt, the harder it was for him to sustain a fusion with anybody, even his closest friends in town.
When he visited Carlosruizite in the otherworld desert, he didn't even ask to fuse.
He also didn't ask when/whether Carlosruizite wanted to come back to Night Vale...or what was going on with the University of What It Is...or why Carlosruizite was steering him away from talking to the oddly-familiar toothy figure who had never been mentioned in any of their calls. He didn't have the stamina to ask for much of anything. (Especially knowing that Carlosruizite has a pattern of promising the thing and then going back on the promise.)
Back in Night Vale, most people shapeshifted into their sharpest forms and flashiest outfits for the arrival of Fashion Week. Moonstone just went with the last body/outfit he'd regenerated into. Didn't even bother to adjust the fact that his arms were different sizes. Didn't care at all what might happen to him.
He got poofed by the Sphere, and couldn't summon the will to regenerate.
Janice keeps the Moonstone in her room, in a box with a handmade nest of blankets. She leaves her uncle's favorite food on the table next to him, reads to him when she's at home, and plays old Western movies on a loop when she goes out. She hopes he'll come back soon.
But nobody makes her any promises.