User blog comment:Silicon Soldier/Xûr's Inventory: August 14, 2015 - August 16, 2015: Gjallarhorn Edition/@comment-26403476-20150816220336/@comment-25916964-20150817133014

@Legit_spartan

Weapon and armor cores are specific to The Prison of Elders, which is why they are some much harder to come across (since they only appear in certain Prison of Elders drops). In comparison, Exotic Shards are not so limited in regards to drop conditions, hence them appearing more often.