Mostly because of the philosophy of the software's author. This was the selling point for me. It is incredibly small scoped by design, easy to understand, and is very explicit in it's actions. It is not as "user friendly" as other inits, though the author is currently working on making it more accessible. It practices separation of mechanism and policy, it attempts to do less than more, and it know when to stop and not introduce scope creep. All by design. I won't ever have to worry about it becoming a system layer and reaching into my network stack, or making policy decisions for me.
I'm sure other available init systems have their selling points - but to be honest, after reading s6's reasoning and design goals I was sold. I'm happy to report that after a year and a half I'm pleased with the decision.
I have long been an advocate against systemd for the typical reasons (overreaching responsibilities, lock-in, lack of choice, software assumption "it will be there", complexity, driven by large corporate interests [see lock-in], being modeled after launched, etc etc). I feel like the chickens are coming home to roost.
That said, if one wants to use it, then use it. However, there are alternatives which I would love to see receive more adoption. I've been extremely happy with my Artix [1] system running s6 [2] init.
I'm sure other available init systems have their selling points - but to be honest, after reading s6's reasoning and design goals I was sold. I'm happy to report that after a year and a half I'm pleased with the decision.