The main reason why people won't use Bitcoin as currency is because its value (I'm not talking about fiat value but the real value) in the eyes of investors keep changing radically. Hence making sellers hesitant to accept Bitcoin as payment and buyers hesitant to spend Bitcoin for payment.
I have created following graph to show the impact of Change in Prices on the people's tendency to spend Bitcoin as payments:
CHART #1
Explanation: The above graph assumes that x number of Transactions (for payment purposes) take place when the price of Bitcoin is y. Now two things can happen, either the price of Bitcoin can increase or the price of Bitcoin can decrease. Both cases can impact the normal behavior of holder. In the case of price rise i.e. increase from y to 2y, bitcoin holder will become optimistic that prices will increase further, this will lead to fall in number of transactions where bitcoin is used for payment purposes from x to x/2. Similarly, in case of price fall from y to y/2, holder may postpone his expenditure or may use other form of payments like general fiat currency for expenditure purposes in the hope that prices of Bitcoin will rise again. Therefore, the number of bitcoin-payments will decrease to x/2.
Overall Outcome: Higher the change in prices, lesser will the use of Bitcoin for payment purpose.
When we talk about Bitcoin prices, another topic comes to play i.e. Speculation! The above chart clearly shows the impact of current prices on Bitcoin's utility as currency but what about the future price? We all know that Bitcoin community loves to speculate. Hence, the speculation also have considerable impact upon the bitcoin's utility as currency. This brings us to our second graph:
CHART #2
Explanation: Unlike the chart #1, future expected price rather have two-ways impact on the number of times Bitcoin is used for payment purposes. Suppose Bitcoin community is expecting the increase in price of bitcoin in coming months. In such case, rather than spending bitcoins for payment purposes, people at large will hold their bitcoins. In other words, if the price increases from y to 2y, the transactions will fall from x to x/2 (it could be anything but I took 50% fall for the sake of simplicity). Vice-versa, in case if people are expecting the fall in the prices of bitcoin, they may rush to spend bitcoins for utilities. This may pump the number of transactions from x to 2x.
Overall Outcome: If the future expected price of Bitcoin is higher, lower will be the use of Bitcoin for payment purpose. On the contrary, lower the future expected price, higher will be the use of Bitcoin for payment purpose.