Not just because it doesn't work as a currency, but because for currencies big swings in valuation, up or down, are no "good performance". Ideally a currency would be completely stable. What $1 buys now would be what $1 buys tomorrow, and what it buys in a thousand years. Of course in reality none of them are totally stable, but the good ones are pretty stable. They move a very small amount, and do so very gradually. They function as a good store of wealth for that reason, and more importantly make for a useful medium of exchange. Since their value is pretty constant, people have a good feeling for how much they are "worth" and can mentally price things.
Bitcoin did well as a speculative bet. If you want to play financial speculation, Bitcoin is a good target as it moves like a very thinly traded stock. That means it can swing bit and make you a lot of money. Also means it can swing big the other way and lose you a lot. So like any sort of speculation, you need to know what you are getting in to and understand the risks.
You BTC promoters can't have it both ways: If Bitcoin is a good currency then it needs to be stable. If Bitcoin is a good investment, then it isn't a currency.