It varies by instance, some are more Marxist, some are more Anarchist, some are more liberal, some are blends of 2 of those. However, the 2 biggest factors are that the Developers are Marxists and thus Marxists have been building communities here from the beginning, and the rejection of Reddit, a highly corporatized social media platform, in order to jump to a smaller FOSS platform, fits neatly with general anticapitalism.