idk if Cinderella Girls >>> Dearly Stars in terms of popularity is true but I wouldn't be surprised...
tbh I think CG is more popular because there are new cards and content... what, every week? Ever few days? It's got something to hook in your attention and make the game more interesting. I haven't "played" CG in over half a year, but I log in every day to look at the new cards and collect a bonus. There are 100+ girls in CG. Whatever your tastes are, you can find a character that you love. Like catgirls? Megane characters? Shy girls? Genki girls? Mushroom addicts? They've got them, even if they only have 15 or so lines.
Plus you can't ever "finish" or "win" Cinderella Girls. Even if you somehow collect 500 stams to trade for your favorite aidoru's SR+, the next week could bring another idol to trade for.
Whereas Dearly Stars was just... one game, and that's it. Once you finish it, there's nothing to do. It just sort of faded out of popularity, not helped by scamco's stubborn refusal to milk it as a cash cow. still waiting on the inevitable 3DS remake/sequel/thing though
tl;dr more fresh content keeps the game alive and therefore more popular than DS