Why is that?
Let's think outside the box: why not sorceror, or nightblade tanks?
A sorceror or Nightblade with 1H-Shield and heavy Armor, can soak up damage as well as a Templar, or DK, no?
(Skill source: http://eldersouls.com/elder-scrolls-online/skills)
SORCEROR
Lightning Form gives nice temporal bonuses to both defenses, and bound Armor would give you a more permanent boost.
Surge can provice a form of self-healing.
Dark Exchange might have limited use for self-healing, though you would be using your stamina to block.
Conjured Ward would give you a nice damage shield
Blood Magic passive might be interesting as a self-heal, combined with Daedric mines: When an ennemy hits you it takes damage AND heals you.
NIGHTBLADE
The Consuming Darkness ultimate ability seems to have potential for tanks: AoE snare and a big boost in resistance for yourself.
The options for self-heals are multiple: Strife (HoT), Siphoning Strikes (Morphed into Leeching Strikes), Drain Power (Morphed into Sap Essence)...
Blur gives a nice defensive bonus for 20 seconds...
The Fortitude passive gives you a bonus to max HP per slotted Shadow ability.
The Shadow Barrier passive gives you a nice bonus to armor and spell resist after you come out of invisibility or stealth (and with Shadow cloak, you can become invisible in combat)
.. Combined with some out-of-class skills, I see some potential..
Let us not limit ourselves to DK and Templar tanks!