Because it is a stupid idea.
It might have a psychological effect but that doesn't suffice to justify such a radical measure.

Why don't we change the currencies' ISO code every time a currency appreciates or devalues.
Because it's important to have stability with every currency.

What do you do when Bitcoin drops down to 30 dollars or even 5 or 2 dollars in the next six months (it's possible)?
Switch back to BTC?

It is a stupid idea as long as Bitcoin isn't relatively stable AND very widely used.
Once it's every other day in the mainstream media such a change could happen without utterly confusing people.
(But we are very far away from that)