This is primarily because Astro was complete garbage until 3.2, being the only job to date that straight up wasn't viable. You literally couldn't clear Gordias until geared due to its underpowered heals. It was buffed adequately enough in Midas to be decent, however White Mage remained the best healer. Granted, Astro had an enormous stigma. Hence why the devs, in a desperate bid to convince players to give Astro another chance, overbuffed it to kingdom come. This didn't make Astro the speed/log healer, it made Astro the undisputed best healer. So much so, that when Stormblood released and those buffs weren't rescinded, there was real consideration of dropping Scholar not White Mage. The latter only retained its prog status after Astro's nerf.
It didn't last, as by arguably 4.3 but undeniably 4.4, Astro was once again considered the best healer. With CU instantly applying, buffs and free mobility, White Mage had almost no answer to Astro's kit. The sheer dominance of Astro can be noted on that website as it remains the only time where White Mage's appeal to less experienced players was barely enough to keep its numbers above Astro and Scholar. Speaking of...
This is straight up wrong. First and foremost, casuals raiders couldn't care less about speed runs. As for logs, the community as a whole practically enforced a "meta". White Mage mains were usually asked to switch, and weren't recruited to statics if they weren't willing. Samurai, Monk and, to a lesser extent, Black Mage all suffered this same problem. You'd see PF constantly locking out spots because bringing a White Mage meant you couldn't parse. Admittedly, this is less of an issue nowadays since that website no longer ranks based on pDPS, but if they allowed Astro to remain overpowered, it would continue to plague White Mage. Case in point, when Ninja was underpowered despite being viable, PF once again locked it out. Hell, the Physical Range were in serious danger of being dropped entirely due to how weak they were in 5.0.
Purposely allowing certain jobs to be overpowered is bad design. You pigeon hole the remaining. Plenty of White Mages don't want to be the "prog healer" who is expected to switch once the team clears. They want to main White Mage.
Now to quote myself from another thread in regards to Astro's cards.
The cards always needed a complete overhaul due to the direction Shadowbringers took. While the system they landed on is, admittedly, boring. Going back to the old cards would literally be "balance or nothing" without massive changes. Simply put, damage and mitigation can never share a slot in this game because damage will always reign supreme.