Because you eventually had to get to each boss by killing actually threatening trash mobs. That, and everything happened in one massive instance. I'd say that is much more akin to a raid than zoning directly into an instance with a boss waiting for you.
Try 10%. The difference between Red Mage and Black Mage is absolutely massive—to the point Red Mage literally serves one purpose: Raise bot. It's completely useless otherwise. People really need to stop dismissing job balance like it isn't a big deal when the current disparity is so one-sided. Certain jobs can perform far below average and still offer more to their respective groups than top tier players in other jobs. A Black Mage or Dragoon who dies shouldn't outpace a Summoner or Dancer yet both can. Just to emphasise this point. The disparity is so bad, triple melee is better than taking any of the Range even if that means a melee or tank will have to disengage periodically.
As for your Zurvan reference. First, I very seldom saw White Mage being locked out. Regardless, you're quoting a period where Astro was absolutely broken; offering essentially an AoE Trick Attack or 20% on a single target. White Mage, meanwhile, did crap damage and had the worst MP economy without question. Sure, it could still clear. But Astro did everything better.
Don't blame the playerbase for wanting a far superior job, which, in turn, makes content like Savage easier to clear. That fault lies on the dev team for not balancing the jobs properly. Case in point, no one locks out Warrior despite it being the worst tank. You don't often see Samurai locked out even if it's objectively inferior to Monk and Dragoon. It's almost as though people care less when the difference isn't staggeringly one-sided. Admittedly, FFlogs' change to rDPS certainly helped. But lets not pretend exclusion has nothing to do with poor balancing from the dev team.