We are definitely keenly aware that people feel better when we fix a problem by buffing something, rather than fixing it by nerfing something. If you look across classes and talent rows in 7.1.5, you'll see the great majority of balance issues were addressed by buffing underperforming talents. There are a few factors that go into this decision though, and it generally has to be handled on a case-by-case basis.
One issue when balancing a lopsided talent row is whether we want the overall power of the spec to go slightly up or slightly down in the process. In the case of Retribution during our tuning adjustments for the opening of Nighthold, we intended to slightly nerf the topend performance of the spec (essentially, the performance of players stacking trinkets/relics/legendaries that all build around Crusade
). I know there's no easy way to deliver that message, but hopefully it's at least worth clarifying that it's not some accident of how we chose to balance the L100 row. Players say "why couldn't we buff Divine Purpose
instead of nerfing Crusade
", for the very understandable reason that the former wouldn't have resulted in a slight DPS nerf. But we were conscious of the slight DPS nerf when we decided to do it this way.
Another factor is that there are sometimes limits on how powerful we want an individual talent to be, regardless of balance within the row. Divine Purpose
are both very strong talents, that have a significant impact on the rotation and playstyle of the spec (and as mentioned, on gear/talent/relic/etc. decisions), even at their current strength. Balancing Retribution L100 at an even higher point would start causing more issues--Divine Purpose
would allow strings of multiple free finishers very often, Crusade
would continue to represent a majority of the spec's damage within a cooldown, and Holy Wrath
would have to get even more bursty to try to compete with the other two. Sometimes we have to be careful about addressing all balance problems by buffing two talents up to match the highest one, because it can lead to a lot of power creep on the importance of a single talent to an entire DPS rotation.