Not to be cynical but you need to take the reddit "sadness" response with a grain of salt. I'm not saying they don't care, we all care about our fellow humans, but they are not actually "sad" here, it's a buzzword to show how "woke" they are, how "in touch." Most redditors outside of NZ and Aus will forget about this in a week and will go on to being "sad" about something else. I would even venture to speculate that outside of reddit these people proclaiming their "sadness" don't even mention how upset they are with the events in NZ to their friends and family.

To answer your other question about why the emotion towards NZ and not Yeme or other places. At least for Americans it has a lot to do with media. American media doesn't really report on what's happening in Yemen or anywhere else in the Middle East for that matter so those stories are non-existant to most of us. I would again speculate that most redditors don't even know that's been happening in Yemen.