Mainly because of tikarability, security and customisation, binary distros don't give you as much control over your system customisability, for example some software might be compiled with features you don't want in the kernel and the source based one just requires some feature disabled so you could get it, with binary ones you cannot. For example if you wanted to get vim with no python support in source based distros you'd just disable it where in binary ones you cannot and vise versa. I also like the customisation of it because some packages won't work without systemD because of the way they were built. Also if the binary built has some odd memory leak you can't make it go away until the next update, but if you compile everything then your binary will be unique. And few more things, like optimisation, every binary you build even better if you pass certain flags to it.