Bitcoin is still a decentralized virtual currency and as such can not be classified as a stable coin.
Bitcoin's price can still be influnced by market speculation forces ...so it's not a stable coin .
For me,the first and foremost reason that makes a bitcoin not to be stable is its own volatility.It will experience a lot of fluctuations from time to time so it will never make a coin to be stable.But if bitcoin will be legalized by the government soon,i think this time bitcoin will become a stable one.
Bitcoin is already legalized on most countries but why bitcoin is still unstable ? The truth is bitcoins unstability is already fixed due to its volatile nature ( like what you said above ) however there are some times that bitcoins value experience a stability though this is only temporary . as soon as people sells and buy , the value starts to move again on un designated directions .