Why is it so hard to get heirloom shards?

I've been trying to acquire heirloom shards for quite some time now, but it seems like an impossible task. Can you explain why it's so difficult to get hold of these elusive items? Is there a specific method or strategy that I'm missing out on? Or is it simply a matter of luck and perseverance? I'm curious to know if there's any insight you can provide on this matter.