Because no such exchange commodity in the history of humanity has ever existed in a vacuum. There will always be multiple currencies, and things resembling currencies.

Existing Bitcoin holders are the worst offenders here. On the whole they don't spend their Bitcoin, except for a little "show off" spend here and there. Basically Bitcoiners killed Bitcoin's future as a medium of exchange by leaning too far into hodl and stack thing, and they only have themselves to blame for Bitcoin now being unable to establish itself as a medium of exchange and forever a Store of Value, treasury, etc.

Up to about 5 years ago there was a chance to steer Bitcoin in the MoE direction, that chance has gone and will never come again.