Liberal democracies seem more conducive to intellectual progress and economic growth than other forms of governance that have been tried so far, and perhaps also to peace and cooperation (at least with other democracies). Political developments that threaten to shift liberal democracies toward authoritarianism therefore may be risk factors for a variety of disasters (like great power conflicts), stagnation, or for society generally going in a more negative direction.
More speculatively, it might be especially pressing to maintain liberal democratic norms in countries like the US, where the ideals have an established history, and there are important technological developments happening (like advanced artificial intelligence) that could impact the long-run future.
For example, science and technology seem to be less thoroughgoingly tied to government and military operations in liberal democracies than in authoritarian regimes (though it’s a matter of degree). If scientists and other civilian actors are more likely to prioritise AI safety vs military operations, e.g. because they are less influenced by national interests, this could be helpful.