No matter what players are going to gravitate towards what they think is the best option, and with subclassing, skill lines no longer have to be compared in the context of classes but rather directly with each other. Now, I'm not trying to argue that Draconic Power is too weak necessarily, and in fact I think that many of the skills and and passives are really strong for tanking. There are plenty of skill lines I'd consider rather weak for tanking but will still use because of one or two unique benefits, such as Animal Companions for better recoveries, self-cleanse, and a nice movement skill, Restoring Light for the auto-block passive and good group healing, or even Storm Calling just for Ball of Lightning and some meager recovery passives.
So why is it that a skill line that is so much stronger for tanking than these also so hard to equip? It's because the value Draconic Power provides is so similar and yet just barely inferior to Winter's Embrace that it is simply outclassed.
The main tools that a PvE tank cares about that Draconic Power provides are as follows:

• Source of Major Resolve - Hardened Armor
  Very strong damage shield for the cost and Major Resolve condensed into one slot
• AoE immobilize centered around the user - Dark Talons and its morphs
  Provides group members with synergy as well
• Strong health-scaling self heal - Dragon Blood and its morphs
  Can be a very powerful self heal skill at lower health, Coagulating Blood is typically preferred in trials settings where tanks have decent enough offensive stats for it to perform better
• Projectile shield - Protective Plate
  Also provides snare and immobilize immunity
• Passive block mitigation
• Passive armor
• Passive healing received

Many skill lines would love to have four very relevant tanking skills... and yet, directly comparing Draconic Power to Winter's Embrace we start to see the issue.
In comparison, Winter's Embrace provides:

• Source of Major Resolve - Expansive Frost Cloak
  Loses a very easy to source attribute (damage shield), in place of one that can only be contributed by one skill and some 5 piece bonuses (group Major Resolve)
• AoE immobilize centered around the user - Gripping Shards
  Gives up a synergy in place of applying chilled. Hard to say which is stronger but certainly not an example of Draconic Power differentiating itself.
• Strong health-scaling self heal - Polar Wind
  Isn't reliant on missing health for larger numbers unlike Dragon Blood (which is particularly useful against heal absorb mechanics), and can heal group members.
• Projectile shield - Shimmering Shield
  Completely absorbs projectiles and returns ultimate. Quite useful in fights with small projectile damage just as a means of acquiring Major Heroism.
• Class chain/teleport - Frozen Device
  Can be better in some situations than a regular chain such as Silver Leash or through scribing skills, but not a common choice
• Passive block mitigation
• Passive armor
• Passively better chilled uptimes
• Sometimes passive Major Maim

Both of these two skill lines are very strong options for a tank to run, with many tools making a tanks job easier. However, because these two toolsets are so similar, the fact that Frost Cloak is generally better than Hardened Armor, and Polar Wind is generally better than Dragons Blood means that there really isn't any reason to use Draconic Power over Winter's Embrace. To reiterate, this is not about one skill like being too weak or strong, and skill lines that are far weaker for a tank than Draconic Power are ironically way more useful to subclass into simply because they at least offer something unique, like those I mentioned at the start. As it stands now Draconic Power really doesn't offer anything that isn't also provided (and in some cases provided better) in Winter's Embrace. Finally, Draconic Power has been a staple tanking line for years up until subclassing and the look and feel of many of the skills are fantastic, I'd just love for the skill line to offer some kind of unique tool that can properly differentiate it from Winter's Embrace so it can have a niche like so many much weaker skill lines do.