I shall try to explain my main points then.
Contrary to what a lot of people believe, it is actually casters that gains the most from gear upgrades. They are weaker in the start because their spell damage, haste, stamina, armor and resilience stats are low, while melee already has a very high stat from scratch - their weapon. Melee also lacks resilience and stamina, but their offense is also much greater because their strikes will hit for a lot more as armor values are lower, health pools are lower and resilience is 0. And the lack of spell haste means warriors can worry less about defense and focus more on offense.
Let's take the example with a warrior and a mage. The warrior's damage scales better the slower the weapon is, along with higher AP. It is here we run into one critical issue - the 2h weapons in the end game lack slow speed. Let's take Apolyon, it sits with a 3,4 speed. Meaning your AP values will be AP/14 x 3,4. Compare this to say a weapon with 3,6 or 3,8 attack speed and you see quite a big loss, because you lose burst damage, even though your overtime damage increases. But in PvP you will not have much use of overtime damage, because every class will CC you for their life, and this means you will often just get 3-5 seconds to do something, which means, you don't have a long time to DPS.
A caster like the mage on the other hand does not run into this issue. The damage of the spells is not dependant on the weapon attack speed, only your spell damage value. Next are the coefficients - they are rather high. Spells like Fireball and Frostbolt will get 1 and 0,8-0,9 of your spell damage value, meaning you lose next to nothing on the margin the greater your spell damage is, so you can stack your spell damage to infinity but you still get almost 100% bang for your buck.
And unlike the warrior, the spell damage will not only make the mage more hard hitting, but due to the haste rating, the mage's overtime AND burst damage will increase. It also opens up options to do more stuff in a shorter time frame. Which means, their "defense" also go up.
The bottom line is that warriors, despite their stats going up cannot match the stat upgrades for the mage, be it offensively or defensively.
I hope this made things more clear.