It's just class balance and content design. In god it begins as physical dps see massive power spikes with new weapons while caster ancients are a tiny upgrade and focuses go 5-10% at best. It's not awful yet though, and god is a wide expansion with some encounters you may still shine on. Oow breaks it as new gear and especially 2.0s shoot physical damage up and its a very narrow expac with just anguish awesome and that zone is resist city. You see shit like warriors in full threat generation and ac loadouts crushing classes like wizard on parses. It's no better in the group game. You are basically trash.
In the meantime, meaningful solo content really deteriorates too, so the appeal of being self sufficient despite weaker in groups or raids loses its benefit. You probably aren't soloing the higher end random god loot, and definitely not mpg group trials or rss named, etc. People get shit done with at least a semi formed group, and casters in these groups blow compared to melee. The spells are more small upgrades, while gear based damage keeps rising exponentially.
In tss the gap is still there but less pronounced. Good new spells, but melee clearly still superior. In sof they finally let casters go hogwild and pets especially become awesome. Rangers become the best pullers somewhere around there 80 or 85 but their dps goes to shit and everyone is getting so many tools it doesn't really matter; imo phase your trueshot armies out by por at the latest.
And a very fun tool set that just gets better.
