User blog comment:FairyRaven/Fairy Tail Chapter 329: Seven Dragons, Review and Commentary/@comment-5871436-20130501012442/@comment-7418318-20130501212036

I wonder if Dragon Lacryma are so rare (and thus valuable) because they can only be optained from the corpses of Dragons. Obviously they couldn't just be picked out, or there would be plenty of them from all the dead Dragons of the past. Or maybe they just don't last 400 years. And Laxus' and Cobra's Lacryma come from Dragons that died long after the civil war period.

Apart from the rarity and price it would also explain how Sting and Rogue managed to get Lacryma that match up with their own learned elements.