Raw Material scarcity - key ingredients used in their synthesis are now more tightly regulated.
Increased demand - more people are Turning to these "cleaner" DHT compounds for lean gains and less E2 sides in contrary to something NPP or Deca.
Crackdown & Labs closures - UGLs are hit hard by regulatory pressure. Especially in asia and europe where alot of raws are sourced.
Shift in preferences: Awarness for water retentive and aromatizing compunds is growing -> people demand more non-bloating options like mast and primo therefore increasing the Global demand for these 2 compounds and as of right now most suppliers cant keep up with.
Hope that helps
FYI in the underground subforum a german supplier (i dont think i can Mention the Name here) still has a BUNCH of Mast E