From what I understand, there is a limited amount of a specific bitcoin, so unlike conventional currency when the authorities need more they are stuffed - currently if the govt. wants more money it just prints it. Problem being the currency (in your pocket) then is worth less as there is more of it about.
Now any currency (bitcoin, dollars, euro, pounds) is only worth what the markets think what it's worth - ie if the market thinks that the controller of that currency is going to dump a load of it or print more of it then the market will get rid before the proverbial hits the fan. At that point the currency concerned drops in value as nobody wants it.
My son has "invested"£3,000 in bitcoin at the beginning of the year - probably worth £30 now Ah well there's always the bank of Mum & Dad