Because it’s necessarily built on bureaucracy and is imminently corruptible. Those ambitious of power are always the ones who rise to power. In a system built on individual wealth accumulation like capitalism, they exploit the system for self enrichment. In a system built on even distribution of communal wealth, the ambition is the seizure and holding of power for reallocation of wealth to themselves and their apparatus. Same types of people, slightly different outcome based on the style of bureaucracy.
Edit for typo.
