Why do street drugs change names?

Well I dont think it's intentional . I believe that the street name changes do to what's relevant at the moment or the effects that it has over your body/mind . Many people use different slang terms for drugss .. The slang terms me and my friends use for marijuana are : Herb , Dank and That Fire .. for cocaine we might say : Yay , yayo , and blow . For prescription drugs such as painkillers & opiates we say : Vikes(vicodin) , zani's(xianax) , oxycotin( straight up oxycotin ) lol .