Because it's catching on like wildfire, a while ago nobody knew what mephedrone was, remember few threads on here with people all like wtf?! is this.
Now headshops sell it everywhere (seriously, every headshop i've been to has had what i'm talking about, dunno if giving the name is allowed so i'll not) and incredible amounts of people have found one of the easily accessible sites...
it'll be illegal before long, cause from what i've heard & personal experiences, it's not very healthy at all. (heard some scary stuff about how hard it is on the heart, and after 200mg my hearts jumping out my chest, and mostly always aches on the comedown..