Well, people wouldn't want to use something as volatile as Bitcoin as a currency. They prefer stability above all else. And the USD proves to be a clear winner.
With investors, stability is not what they look for but volatility and solid growth over time are what they most wanted. Bitcoin has it for them and it has shown this amazing growth over years since 2009.
Even an older investment asset, store of value, like gold has volatile times in its history. So it does not make sense to ask Bitcoin only has stability without volatility.
Besides, BTC's deflationary model encourages people to hoard it like a store of value. If Bitcoin had stable market prices and was inflationary, El Salvador's efforts would've been a huge success.
It's hard to succeed in national scale or world scale with anything centralized, mandatory enforced by goverments. Citizens are massive enough to have very diversifying social-economic statuses, hence not all of them can get benefit from governmental policies. Give citizens freedom and choose what work best for them, it's a more realistic way to gain success.