That's easy. If you look at the picture, you see that Debian has the most derivatives, so clearly they are doing something right. Anecdotal evidence also suggests that Debian tends to be the stable base that "just works" so that derivative distributions have plenty of stuff they can break.
ps. I want to point out though that Debian "stable" tends to be (somewhat by design) sort of obsolete by the time it's designated stable and unless you're running servers you're probably better off using "testing" (the "next stable") most of the time. Obviously it's not "audio optimized" so some manual tuning might be required, but ... at this point Debian has at least some 30 years of track record with being the distribution you fall back to when you're tired of the latest "hype" distribution breaking stuff again.
pps. Also Debian "unstable" doesn't mean your computer becomes unstable and constantly crashes. It just means it's what other distributions call a "rolling release" so packages can come and go and sometimes thing might break if what you need gets dropped... not great for business, but generally it's fine to use at home as well.
