it's the combination of using epic materials twice as fast as legendary and the fact that legendary have a much higher drop rate from the high level faction crypts. farming lower levels can fix the problem, but between the reduced amount of materials and the lower level glyphs, it's not really worth it imo.

Farm the highest levels you can and only craft 5-6 stars and you should be good. 

Rufus, please see the picture above.  I have 1200+ legendary materials and <200 epic.  I've been farming primarily 20/21, resulting in the imbalance.  As carpe_pisces suggested, to farm lower levels would result in no high level glyphs.  

I also farm exclusively 21 and 20 and have no shortage of epic materials, as they do drop as well from those stages and in much grear quantities than lower stages, so if anything just persevere and hopfor better RNG. 

I ran into the same issue but it was from crafting 4-5* gear just to sell.

Login  rewards  give  legendary  materials.  I  always  have  plenty  of  legendary  pieces  but  never  enough  epics.  Legendary  pieces  drop  too  much  from  high  level  crypts  compared  to  the  epic  pieces,  knowing  how  you  need  both  epic  and  legendary  pieces  to  craft  Perception  and  Resilience  sets.  Maybe  add  100  epic  materials  alongside  the  50  legendary  pieces  in  Daily  Login?