An article in the Washington Post asks
Why are liberals so condescending?
The article describes several types of condescension, but never gets around to answering the question. At least part of the reason is that liberalism is a mental condition rather than political philosophy. In particular, liberals are obsessed with a vision of themselves as morally superior to everyone else due to the inherent virtue of their policy positions. This allows them to feel good condescending to others. If they were forced to seriously consider objections to their notions, it would introduce doubt about their superiority, shattering their self-worth.