A major problem with BTC as a reserve currency is the way it is generated (mining) and reclaimed (fees). An entity with significant computational power would be able to maintain significant control over the reserve currency. It's feasible that early adopters, then, would be holding significant percentages of the reserve currency. What would motivate later adopters to convert when they have perfectly viable reserves already (gold, USD, etc) that's more equitably (well, from their perspective at least) distributed?

Second problem, it requires a major shift in the motivation of many adopters (anti-fiat, anti-fractional reserve banking).