Why do you think the Sakura series (Sakura Angels, Sakura Beach, Sakura Swim Club, Sakura Fantasy, Sakura Spirit, etc...) are so popular? Are they related? And have you played any of them?
Pretty much fanservice and the fact that they are pretty cheap in Steam, I think. For OELVNs, the Sakura series also has pretty high quality art. I never played any of them (thinking of at least starting Sakura Fantasy just for the yuri, but have a feeling even that's not gonna end well even if I like yuri, lol), but to my knowledge, no, I don't think they are related.