text: first it's the speed;  hddrives [if that what u refere at secondary storage] are the slowest things in an computer coz they have mechanichal parts included. Ram dosn't uses any type of mechanics it's just a small intagrated circuit board including millions of transistors and capacitators.\n  RAM has to be dynamically refreshed all of the time or it forgets what it is holding, hard disk store it's  information magnetticly into platters and dosn't need any periodic refreshing.\n  Size ..  due to limitation of technology rams can not have the same ammount of mb's as a hdd