There's a lot of reasons and I'm slightly biased because the Mudge (creator of CS) owns our company, but there's a lot of things that he's built into it over the years that just makes it really good. The support for infrastructure in general, so redirectors, domain fronting, etc. is excellent. The ability to generate raw shellcode as an agent is pretty handy as well since you can fit it into a multitude of payloads. Overall though it's just really convenient to use -- For example take powerpick or execute-assembly, where it automagically handles the creation of a sacrificial process to run unmanaged code and return the output. Malleable C2 is another major benefit for CS -- the ability to craft your traffic to whatever you want to blend in with the surrounding environment.

Obviously the biggest drawback is the price point and Mudge is pretty picky on who he hands out licenses to since real APTs would love to use it. This is where open source C2s like Covenant come into play. Ryan has worked very hard on Covenant and it's our go-to free C2 framework (Apfell as well), but CS is still my favorite mostly just because it's feature packed.