Why, and why not? We are not economists nor currency experts, so interested in the real reasons that this may be true.
That this property is very slow and difficult to achieve should be obvious. It's that chicken and egg problem that Bitcoin has been gradually solving, that's characterized by massive volatility. That volatility is a problem for those who are primarily interested in predictably transferring purchasing power across time, and its source is not yet having enough market liquidity (and in turn, market depth). If Bitcoin manages to achieve sufficient liquidity for large scale finance, then where's the incentive for the big players go through the slog of bringing another one up to speed, unless it offers some majorly useful technical breakthrough? And then wouldn't it be a whole lot easier, and financially less risky, to just upgrade Bitcoin?
Everybody acting like squirrels running from copycat to copycat is not conducive to very desirable stability, and so a Nash equilibrium is for large, thoughtful, entrenched interests to make up their minds and get on with business.