A currency becomes too volatile compared to other currencies, which makes it very difficult for us to use it for everyday payments.
Not only bitcoin is volatile because judging from the current rate of fluctuations on my nations currency, then I would also say that country's currency is also very volatile yet its been used on a daily bases and one might say that the volatility in my local currency isn't obvious but I'm sure those who use it in importation are already crying from the devaluation as compared to the us dollar.
One major reason why people are still very committed to bitcoin in general is the volatile nature because it where all the profits as well as the losses lies and if you take away its volatility then I'm not sure if most persons would still stick it to.
But does your national currency increase or decrease by 10%-30% in 1 day? All currency pairs between countries volatility with each other, my country's currency pair against USD also volatility every hour, but that volatility is insignificant compared to bitcoin. And people can also take advantage of that volatility to make profits, which is the forex market we have today. But that doesn't mean volatility between currencies and bitcoin is the same. I have never seen a national currency increase 10% or decrease 30% in one day against the the USD.
Of course, up until now we have been in the market for profit so volatility is extremely necessary, but if we talk about bitcoin becoming a currency, volatility is a barrier.