bitcoin is no a legal currency because its not paying taxes in the government, the number one reason why if btc is paying taxes in evry transaction maybe it will a legal and another reason why its because some people uses btc in a fraud way.
There may be tax reasons as you say. But there are other reasons as well. For example, the inability to manage and control Bitcoin in liquidity operations is easily a tool for the sale of arms, drugs, money laundering, corruption, etc. These are potentially dangerous issues harm to social security. In general, the government does not recognize Bitcoin as legitimate because they believe Bitcoin management is not possible.