Its this way because it is the strongest stat, for all classes, for all builds with the sole exception being those who rely on overpower.
Every class can go from +20% vulnerable to +100% vulnerable, by using the glyph and priorizing it on your equipment.
This also makes the largest difference, because it is a 3rd multiplier.
This means you are using your previous damage more efficiently instead of diminishing your own values by sheer size.
Assuming you would be at 100% critical strike change, your base damage would be
100 (total sum of your damage modifications)
+50% (critical strike damage base) +20% (vulnerable) = 180
Now looking at the strongest roll of critical damage vs vulnerable you would change to
+67.5% +20% = 201 or
+50% +34% = 201
But hey, you arent going to have 100% critical strike chance, so vulnerable wins at this point 
You were seeing a 3% chance happening.
Should be happening much more often than finding a good item.