For a very long time in the US, the only things that were taxed were physical, tangible things like physical merchandise or real estate. Physical things are a lot easier to track than abstract numberical quantities in various accounts (which is what "income" or "capital gains" really are).

The income tax was really a temporary measure to pay for World War I. (Spoiler: It wasn't temporary.)

If Bitcoin will make accounts anonymous and untaxable, what's wrong with going back to a tax on physical things?

The 16th amendment was ratified before WWI began, also four years before the U.S. entered the war.