Perhaps, in the early stages, bitcoin grew and was popularized due to the fact that people used it as a currency, and not as a speculative instrument, I will not argue.
I would actually say the opposite; there are more ways to spend bitcoin now as a currency now than there was ten or so years ago due to emergence of more merchants and projects such as lightening network.
But now the value of bitcoin is growing due to the fact that more and more various IRL spheres and companies support bitcoin. But this does not mean that the more companies support it, the more often Bitcoin is used as a means of payment, otherwise we would not see a constant outflow from the circulating supply, and, consequently, the price increase.
So many factors affect the volatility of bitcoin, that it would be impossible to pinpoint just one, but the most salient imo, should be the features it was built with, which attracts companies and individuals.