That's a very good question. The reason is that Cuba is a socialist nation, and the US is ideologically opposed to socialism, because it is a threat to the interests of the wealthy ruling class. Due to the popularity of the regime and communism itself in Cuba, it was known by the US that the only way to effect regime change was through economic pressures:

As this document outlines:
while as adroit and inconspicuous as possible, makes the greatest inroads in denying money and supplies to Cuba, to decrease monetary and real wages, to bring about hunger, desperation and overthrow of government.
Of course, this didn't prevent them from waging a campaign of US state-sponsored terrorism in Cuba, nor from proposing a CIA-backed 9/11 style false-flag terrorist attack against US citizens in order to gain support for a war against Cuba.
Sorry, are you expecting the government, which is owned and controlled by the ruling class, to make legal changes which would go against their own interests? Haven't you been paying attention?
If you want change, there's only one way for us to get it, and it's through a social revolution.