In the forefront of the party’s political tasks is the work in the trade unions for its development and strengthening. The criterion, by now common to both fascist and democratic union politics, of attracting the workers’ unions among state bodies, under the various forms of its regulation with juridical framework, must be fought against. The party aspires to the reconstruction of the trade union Confederation, fully independent from the direction of State Offices, acting with the methods of class struggle and direct action against the bosses, from local and category claims to whole class interests. Workers belonging to different parties or to no party at all can join the workers’ union; the communists neither propose nor provoke the division of the unions due to the fact that their governing bodies are wholly in the hands of other parties, but they fully openly proclaim that the purpose of the union is completed and integrated only when the political class party of the proletariat is at the head of the economic bodies. Any other influence on the proletariat’s union organizations not only takes away from them the fundamental character of revolutionary organizations demonstrated by all history of class struggle, but makes them sterile for those very purposes of immediate economic improvements, by making them passive instruments in service of the interests of the bosses.

TLDR; It's because the unions were fully integrated into the bourgeois state machinery and thus has lost their class independence.