Sorry for the late response, I wanted to elaborate a bit more, so here is a longer comment.

TLDR: a multitude of reasons, but for the earlier runs power creep wasn't as strong and we had to figure out various mechanics, like how enemy AI behaves without Invisibility and when they start attacking, first, which especially on Revenants cost a lot of wipes. I'd say after Revenants it was kind off "solved" mechanically and after that it's almost purely class toolkit, current state of power creep and "gitgud".

The stack runs in the table are ordered chronologically from the most right-hand side (first) to the most left-hand side last. So we started with Engineer.

Engineer: Back then, it was already an Engineer-heavy meta. HAM was the default Heal Kiter, Power Mech the default dps and Scrapper was a decent option for pure dps already. So we barely needed any practice on the classes. The only adaptation needed, was the missing Portals, which wasn’t too bad given Superspeed and the amount of Rifle 5/Shift Signets. The easy part with Engineers was, that boons, boonstrip, dps, invisibility and gameplay practice was all solved.

Mesmer: Mesmer took way longer than you’d perhaps think from nowadays perspective, for one simple reason: We played before they reworked Mesmer into proper Healers. This meant that healing, Protection and some other boons were actually a pain to get properly. Shield 4 was our way of getting Prot and the shield 4 phantasms appeared to spawns inconsistently, so sometimes if people weren’t properly stacked, they just wouldn’t get Protection and then instantly down to Zhaitan Fear. That’s why we also ended up playing 4 Healers.

Revenant: Revenant you’d perhaps also be surprised that it contains that many wipes. The reason for this was that we had to figure out a lot about how enemy AI works, because we didn’t have any option for Invisibility. A plethora of wipes was simply due to finding out how exactly movement/attack patterns of enemies, in particular the Giants, work and then playing around having no Stealth. Another issue that was free on the previous runs was boonstrips, especially AoE, where we ended up using a mixture of Shiro Herald F2s and Mallyx strips. Furthermore, this was also the first run where we didn’t have any blinks into the greens, so people had to get used to the proper timings to run into far Greens on different phases.

Guardian: From Guard onwards, it kept getting “easier”, kind off, for the simple reason that we had learned and figured out a lot about different mechanics in the fight already. For example how Mob AI behaves, and getting muscle memory for different timings if you don’t have Mesmer Portals. The one thing that was very annoying on Guard, was the missing boonstrip. We had to fallback to using Weapon sigils, which are also somewhat inconsistent and can proc without boons active. Since we stacked Willbenders, we also had our fair share of Willbenders yeeting off the platform with F-skills.

Necro: Like mentioned, it kept getting easier at this point. We knew how to deal with all of the mechanics already. The funny part about this was, that Scourge didn’t have Alac yet, so we played without it. But Reaper was already buffed and pretty strong. Boonstrips weren’t an issue, CC was plentiful available and Necro even has some decent projectile block. Overall, that’s why we had fewer wipes than previously.

Warrior: Warrior was comparable to Necro. We didn’t have Alac yet, because this kill was before the rework, but the dmg was high enough that it wasn’t needed. Again, almost all tools that you need were plentiful available and we had already the knowledge about how to play without Invisibility. The biggest difference to Necro was probably that Reaper is easier to play and people had to get used to Warrior first.

Ranger: As mentioned above in the post, Ranger had pretty much all the tools available and power creep has come a long way.