1- Because the damage reduction (or HP boost) is easily made up for by our CDs that also happen to be as strong or stronger than the tank stance, making the "damage cut" in the damage we deal not worth it.
2- Enmity is a non-issue once you establish a lead. You only need "1" more enmity for the boss to attack you instead of the guy that's less than you by 1. Once you establish a lead so their next few actions do not beat you by 1, additional enmity is redundant. We already have a billion tools to ensure that tank is always ahead.
3- Tank stances already have a penalty of: "Not being in DPS stance" which denies tanks good offensive tools (Fell Cleave, Added hit on auto-attack, Bloodweapon), why is it compounded by an enforced additional -20% and barely ever added "good defensive tools"? Heck, PLD generates Oath Gauge in Sword Oath ~40s faster than in Shield Oath lol. How is this not hilarious?
4- Tank stances add nothing to gameplay to when it comes to decision making (to make them interesting). As mentioned in points 1 and 3, it is never worth going into tank stance for a net-gain of any sort (due to CDs doing better at no penalties) nor do we have enough locked behind stances to influence said decisions (unlike BRD that has very neat tools behind their song-stances).
5- This one is the most important one in my opinion... Stances are in place to PUNISH >ME< for doing "MY JOB" of holding hate and reducing damage taken when I can achieve the same result without them. No other role is punished for neither doing their own job or helping the other roles. SE already went through the trouble to ensure healers do not suffer from a "DPS stance" and BRD/MCH don't lose 10~20% DPS for using MP/TP regen abilities by giving them oGCD refresh and tactician instead.
So yes, people will dislike stances as long as they stay the way they are. Read: "BADLY DESIGNED".
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On topic... I saw somewhere a leak of some of the removed abilities. Dark knights are losing Dark Arts. I hope this change doesn't make DRK just another "PLD"... But I understand the decision as it is a source of design problems. Wild Fire is also going bye bye for the same reasons I believe.
I hope the list isn't exhaustive, because I didn't see Flash nor Unleash on it (but I saw heavy thrust and Thrill of Battle on it). If they stay in the game, they NEED to be changed because as they are they are just terrible moves.