First past the post is equivalent to government

Anarchists want to liberate themselves from the state and the state (generally) seeks to attach itself to those who seek to leave. Democracy is the primary means by which people who do not like the state are able to liberate themselves from it but if the voting system is not proportional this weakens this ability. If the voting system is not proportional anarchists will find it more difficult to remove themselves from the state when they vote. Voting is bad for the government but fptp is less democratic (less proportional) which means it protects the state from the liberation of the people.

With either direct democracy or pr (proportional representation) the voters are able to some extent to escape the state which means that these systems are better for anarchists. The fptp system protects the state and so it is preferable for the state to have.

First past the post is associated with the state.