First, countries are not obliged to take into account science when drafting laws, so on fundamental level answer is that bitcoin is currency because US government decided that it is a currency regardless of what science has to say on the matter. US government could declare that, for the purposes of deciding your tax liability, alcohol or jeans counts as money so you have to pay taxes on any trades involving alcohol or jeans regardless whether from economic perspective they are barter or not.
However, from economic perspective:
Money generally regardless of what currency we talk about have to have the following three properties:
- Serve as a medium of exchange
- Serve as a unit of account
- Serve as a store of value.
If there are vendors and other people that are willing to generally accept bitcoin in exchange for goods and services in exchange and they use bitcoin to measure value (e.g. quote prices in terms of bitcoin etc), then bitcoin satisfies 1 and 2. This would be so even if it is generally accepted only in some more narrowly defined economy. Moreover, bitcoin and generally most assets out there, can be used as a store of value. So superficially bitcoin can be considered to check 1-3.
Furthermore, Bitcoin or anything else can be money at some point of time, then stop being money at some other point of time, and later start again being money depending on whether bitcoin checks the 1-3 prerequisites to be money.
Bitcoin is considered to be money by many economists (e.g. see Hazlett and Luther 2020) as it seem to check all three requirements (even though some of them only so so), and consequently it can be considered to be money (or in foreign exchange rate terminology currency). This being said, there are also economists arguing that actually Bitcoin is just an asset as it is a poor medium of exchange due to low acceptance, and also poor store of value (e.g see Yermack (2015) Is Bitcoin a Real Currency? An Economic Appraisal in Handbook of Digital Currency by Lee and Chuen).
Consequently, to sum up there is no clear cut consensus on this issue in economics yet. Also, something can be a money at one point in time and cease to be money at another. However, US government can decide to pretend bitcoin is a currency even if there would complete 100% consensus among economists it is not currency. Governments are not obliged to follow science when they draft their laws.