And the main reason they don't consider it currency is to impose a tax that discourages its use.
Without capital gains, if Bitcoin had ~0% inflation and the US got back up to 5%-10% inflation, then Bitcoin would become a better store of value.
However, with capital gains, the US would have to have significant inflation, around 20% or more, before Bitcoin would really be competitive. (I'm assuming there are significant transaction costs related to the low acceptance of Bitcoin)
In this manner, the US can (1) gain some additional tax revenue, and (2) prevent Bitcoin from becoming a competitor to the national currency, which every nation wants complete control over.
Without capital gains, if Bitcoin had ~0% inflation and the US got back up to 5%-10% inflation, then Bitcoin would become a better store of value.
However, with capital gains, the US would have to have significant inflation, around 20% or more, before Bitcoin would really be competitive. (I'm assuming there are significant transaction costs related to the low acceptance of Bitcoin)
In this manner, the US can (1) gain some additional tax revenue, and (2) prevent Bitcoin from becoming a competitor to the national currency, which every nation wants complete control over.