The issue is the clutter of Strengh and Agility two-handers and the limitations it puts on overall loot tables and connected RNG of said weapons dropping. Allowing the major stat of the item to be able to shift from Strength to Agility or vise versa you open up multiple specs to the same item rather than that item suffering from Spirit Plate Syndrome (SPS). Two handed Strength Maces can be used by Feral Cats, Agility Polearms can be used by all three plates classes for their five combined specs, and that lone Strength one-handed axe that keeps dropping can be used by the combat rogue who is still using last tiers weapons.
Along with this concern is the elusive Strength ranged slot. Honestly just shifting us to a Relic slot would be the easiest solution since we would share the same item with our plate brothers for both tanking and dps, but we would need to be compensated in some way for loosing shoot/throw.