Because Savage Defense in cata was useless? It was a static number considering youre always vengeance capped, it was still great.
Useless? Not entirely.
Poorly scaled, needed constant band aids, and utterly pointless on many challenging fights? Definitely yes.
SD suffered anytime you took more hits in a set time frame (multiple attacks, haste, etc), the damage per attack began to greatly exceed the bubble size, or we were unable to attack consistently.
Zon'ozz was a classic example of pain for Druids that did NOT affect the Block Tanks in the same way. His self-buff hasted his attacks (less likely of having SD up for his hits) and made them hit harder (SD mitigated hits didn't scale with him). On the flip side, Block capped Tanks continued to Block even if he sped up his attacks and because they chipped off a percentage, they scaled with his increasing damage.
Warmaster had similar issues that were worsened simply because the two-hit combo of Devastate/Melee were enough to global a Druid at 0%-5% without a bit of luck or a lot of external/self CDs.
Blood kiting? /chuckle
But on the other side of the fence you had things like Hagara where our baseline mitigation and SD bubbles trim the pain of her Focused Assault, and our passive mitigation and amazing CDs lined up very nicely with Madness Impales.
Cata SD was a relic of Wrath SD where the blocked amounts weren't anything special at all, just like real Block. It didn't scale, it didn't work very well, and it didn't significantly impact our chances of survival. That's why while it was mathematically pushed to focus on Agility (for improved SD and Dodge increases), you could stack Stamina and be just fine, albeit more of a drain on your Healers.
If our FR was static and didn't scale well or scaled like SD did in Cata, we might as well not use it ever in competitive play and then simultaneously piss off every group we get in 5mans as we race ahead of the other four, self-healing and killing everything on our lonesome.