The way it works is that there's a series of dice rolls: dodge > crit > block > parry. If one passes, it doesn't continue. So a critical strike can be neither parried not blocked, and a blocked strike can never be parried. People did tests in that other thread to figure out how the roles worked, recording thousands of hits with various cooldowns active to see how it played out.
It's because for the first wing of Alexander, there wasn't nearly enough physical damage for it to make a difference, and the main thing that was killing groups wasn't dead tanks, but failed DPS checks and boss enrages. Progression groups needed every scrap of DPS they could find, and that meant tanks running content with 15,000 HP and as much Str/Det/Crit/SkS as they could get their hands on. Simply, the encounters were not designed to require heavy mitigation, and were designed to require higher DPS than the groups could put out at the iLvls they were attempting with.
The scaling issues that were highlighted were also seen as a kick while tanks were down. Parry strength used to be affected by other stats, and while it scaled more poorly in 2.x, you could still achieve high parry rates and %s by combining it with other pieces of gear. In 3.0, the strength was changed to a flat 20% mitigation on physical hits, and the scaling was determined to not be that much better than it had been, piece for piece between 2.0 iLvl 70 @50 and 3.0 iLvl 170 @60 gear (gets slightly better at 200+, but it's hard to compare tiers to one another when they used to do things so differently). And given that people thought parry was bad in 2.x, the notion of parry being bad was basically universally accepted once it was fully evaluated for 3.0 (with a few exceptions). Even if it's the best mitigation point for point, it's still considered a poor stat, because it's still not very effective, and is arguably worse than it was before.