Trust and security.

Basically everyone has complete tust in bitcoits not disappearing overnight and the security in which bitcoins not becoming worthless in comparison to other currencies (usd, gbp).

Compare to the Euro - people are losing trust in the euro, it could disappear and force everyone to revert to a localized currncy and the security of its worth is depreciating in comparison to other currncies (see the eur/usd swap rates over a three year period on forex sites)

And as for the taxes thing, if bitcoins became the next usd by some miracle governments would simply place high taxes on goods and services to compensate for the decrease in income tax revenue.