Because workers are a commodity to be traded. That's where value in capitalist market is created. Workers, as ancillary tools in the service of capital, create value. So therefore workers need to be prohibited from organising into a union as this will undermine workers commodity status and cause a crisis to capitalist production. A very fragile system based on the exploitation of labour which is not paid for the value it produces. This labour needs to be coerced into having exactly no power over what will be produced or in who's interest it will serve.
