Willpower's a better, more effortless Regen than Regen.
WP gives less TOTAL Regen, but tosses decent Resist and Defense in.
So there's less "I'm a GOD! I'm a GOD! I'm a *ROAD PIZZA!*"
The reason why Regen is unavailable for Tanks is because Tank HP numbers are so through the roof that even a baseline Regen Tank would START at the immortality line.
A middling Brute can pull down 61.59 HP/Sec out of IH or MOG. Note: Including all +HP Accolades, with Common IOs used in everything but IH.
IH alone is 172.99%
With a bit of tweaking, you can push to about 182 without searching for +Regen/+HP set bonuses.
And you can tweak to get it up to 130s Recharge.
Now imagine that with 20% more HP.
Then toss in "Chance for +Absorb"
Plus Numina and Regen Tissue uniques.
Utterly redonkulous.
So, you simply cut back Regen levels IN REGEN? And leave Tanks with an "inferior" setup?
Or do you diversify, making up that lost Regen with things like +Resist, +Def or +Absorb?
And what do you wind up with?
...Willpower.