If it were guaranteed that the price of bitcoin would stay the same as it is today, would you still buy it? My guess is, the answer is no. But if its value lies in being a currency, price stability is essential. And if your answer is no, how is it then not just a Ponzi scheme? In other words, everyone is buying it on the hope of someone paying more for it in the future, not to use it as a currency.
The answer is very simple, if bitcoins value will not be moving or not volatile then no one will invest because it was not profitable, volatility is important in order for us to earn profit.