To answer the OP:
1. Tanks have been nerfed 3 times in regards to the damage dealt, along with their complexity. It started in Heavensward when they first shifted Tanks from relying on STR for ATP to 45% of STR + VIt for ATP, echoed into Stormblood moving back to STR and making tank accessories VIT only, until a little later on after outcry/people using level 60 accessories for damage until Sigmascape.
2. By trying to make Tanking more inviting to general players, rather than doubling down and making it feel like home to people that love it, SE has erred greatly. Psychologically, most people do not like playing Tank. The reasons for this are myriad and vary person to person.
3. The new job being a tank only works for a short time to increase the tank population. People will level it out of love for its fresh look and aesthetic, but very few will stick to tanking just to enjoy that aesthetic.
4. Tanking involves some level of accepting responsibility within a party dynamic as well as making decisions. Most people in this day and age are horrible at making decisions, even if they are as small as, "Should I pull one group or two?"
5. In high end content, other players look down on tanks and treat them like numb skulls, unless the tank is exceptional. This usually involves the Tank having proven themselves in roles other than tank, putting the group into the mindset of, "He's an exceptional player."
6. There's no longer any knowledge dynamic or race for enmity. It's a binary of, "Is stance on?" "Yes." You're tanking. "No." You're not tanking. This means an entire branch of joy from actually being good at managing that, though it wasn't much harder than it is now, was destroyed.
7. Enmity Combo vs. Damage Combo removal. Ties into #6. Before you had a choice on tank between doing more damage/gaining more resource, or edging yourself up a little higher on the enmity table. WAR had a weird thing where, depending on a lot of factors, the enmity combo did more damage(and back in Heavensward it just did more). This let you make a a risk/reward choice that felt good if your choice paid off.
8. Big changes to two of the older tanks resulted in longtime players of those tanks no longer enjoying them. Ties into #2. No self heal on Inner Beast/Inner Chaos is criminal. What was once a 300% of damage dealt heal no longer heals at all. While people complained about Dark Arts, it was a cool animation, and originally allowed DRK to buff itself in unique, albeit sometimes useless ways. The fact that it was a CD that could be used for damage or for mitigation enhancement is why it was fun. There was also the fact that DRK could self sustain on big pulls, the larger the better with DA + Abyssal Drain spam. Self sustaining on the wall to wall pull right after Magitek Scorpion in Ala Mhigo was the most fun I had on a tank in a dungeon.
There's more, but there's also a character limit.