I think it's easy to understand sarcasm when you know who you're talking to, such as a group of friends or coworkers. But when you're part of a huge online community like hn that discourages humor, it's pretty easy to understand why people fail to recognize it - this community punishes sarcasm more often than it rewards it. And most of us are not talented enough at writing that we can make it easily understandable to a broad audience.