Shadow artifact v. Disc artifact - What am I missing?

[Ammonite]

Why does almost every priest (regardless of their spec) take the shadow artifact?

Shadow artifact at ilvl 34 it gives:

• 25 Int (MH) + 7 Int (OH) + 5 Int (Enchant) = 37 Int
  
• 6 Stam (MH) + 3 Stam (OH) = 9 Stam
  
• 4 Crit (MH) + 1 Crit (OH) = 5 Crit
  
• 4 Mastery (MH) = 4 Mastery
  
• 3 Haste (OH) = 3 Haste

Disc artifact at ilvl 34 gives:

• 37 Int
• 13 Stam
• 9 Haste
• 9 Mastery

Even after enchanting the off-hand, the net loss/gain for taking the shadow artifact is:

• -4 Stam
• +5 Crit
• -5 Mastery
• -6 Haste

You can enchant both the shadow MH and the disc staff with the +11 spellpower, so that shouldn't be a factor. I am overlooking something? Taking the disc artifact seems straight up better but almost nobody does.

 

[Mnemonic]

You’re using the level 20 stats shown in /inspect and armory. The actual offhand scales to 34. Also, the enchant scales up in bgs.

 

[Ammonite]

You’re using the level 20 stats shown in /inspect and armory. The actual offhand scales to 34. Also, the enchant scales up in bgs.

Ookay, got it. That makes sense then, thank you