Single-splat games have a lot of benefits. They’re easier to run, especially because they tend to be smaller. Smaller size and tighter focus can make for more personal storytelling and more individual attention for the player base. (Not that they always do, but they can.) And you can have a bit more freedom in terms of how you construct your game world – if you want your Werewolf game to have vampire bad guys who aren’t really like the Camarilla and also aren’t really like the Sabbat, it’s an awful lot easier to do that when you don’t have to totally rearchitect a PC Vampire sphere in order to do that.
Kitchen-sink games are harder to build and run. But they do have an advantage in that all-important “engagement” factor: because they tend to be larger, it’s usually easier to find someone to RP with when you feel like playing. Even if it’s someone from a different, possibly hostile sphere who you can’t talk shop with, there’s a lot to be said for making it easier to get to the “roleplaying” part. And while yes you could have a single-sphere game that has that level of engagement, I think it’s easier with kitchen-sink games just because you’re maximizing the number of people who your elevator pitch will grab onto.
While I might say I prefer the former, and I’ve had a good time on some single-sphere games, according to my revealed preferences I value being able to find RP easily more than I value the potentially more personalized and more individualized experience of a single-splat game.