IMO. The problem is that in a team based game, high defense can be avoided by targeting lower defense allies. That is - defense only works as a tactic if the whole group matches (or nearly so, your investment). You need something that incentivizes enemies to attack your high defense character or the defense is basically worthless. For a caster that incentive might be casting a strong concentration spell. For a melee character it might be grappling the BBEG.
Because of the above the defensive traits that are highly valued are the ones that can prevent you from being disabled in combat (Resilient Wis for better wisdom saves) - which does tend to get recommended by whiteroomers.
Right, in a team game (which, generally, D&D is) defense takes on a bit of a different meaning. It's not about individual defense but about making sure the party members best able to take damage/effects are the ones being targeted.
So for ex. spells such as Compelled Duel are excellent because 1. They actually make enemies reconsider targets 2. They're a bonus action, so allow the caster to do something else too.
And for fighting styles IMO interception and protection have a bit more value than defense, because the two former actually take damage away from targets you don't want taking it.
Too many people think "tank" means difficult to hit. But that's not it, a tank that's overly difficult to hit can't do its job effectively because enemies won't bother targeting it.
Blur requires an action, only affects yourself, and enemies can choose to not target you after it's up (limited incentive to attack you vs someone else). Blur may end up preventing no damage (or more), does prevent you from concentrating on anything else, and requires a resource.
Contrast with Healing which leaves your concentration slot open, often just requires a bonus action, can target anyone in the party, and the slot isn't spent till it's actually needed (meaning it's never cast for no affect). *And that's before we get into whack-a-mole healing.
That's why Feats like inspiring leader and abilities like the twilight aura are so good. Preemptive of any fight and provide great HP with very little effort during combat. This also ties right into the group defense goal, vs. Individual.
That's why Blur isn't valued highly.
Most valuations of defensive options are highly DM dependent and depend mostly on how the DM runs enemies in combat. Do they always target the turtle tank. Are they never willing to take OA's to engage another target. Etc. But if given a particular set of enemy tactical assumptions the right whiteroom handles defensive abilities just fine.
My takeaway from defensive style is different. Characters that go sword and shield tend to take duelist because it gives 'enough' damage and defensive style gives little defense. Characters that go Great Weapons tend to go defensive style because the damage from GWF style isn't deemed high enough compared to the +1 AC they can get. Maybe players should value the damage vs the defense differently but I happen to think they've already correctly figured it out.
Defense is a "good enough" style if there isn't a better one. IMO, if Tasha's is allowed most people who would take Defense would take Blind Fighting instead. More situational, but just SO MUCH better. Heck, I'd consider taking Blind Fighting over Dueling depending on campaign.