Currently, a large portion of tank damage (roughly 20% in AoE and 40% in ST, depending on spec) comes from external sources such as cantrips and trinkets. There are two general schools of thought on tanking - the older mentality, which has been generally de-emphasized since Legion, is that tanks exist primarily to soak damage and survive, while the more modern one is that tanks should also help contribute damage since survivability can come from proper rotational play.
Tank damage contribution is also important both in Raids (who hasn't had sub-1% wipes) as well as Mythic+ in which beating the timer is the core gameplay. This change negatively affects both schools. Holding agro, especially against the on-pull burst of some DPS specs, has consistently been an issue the threat bonus was nerfed after Legion to prevent tanks from permanently kiting. This is generally less of a problem in Raids because taunt exists, but it has been a sore point in Mythic+ where managing threat on multiple enemies can be problematic.
By making cantrips and trinkets deal less damage for tank roles, it does shift power back into the base kit, but the kits are not tuned well enough to handle it. As an example, Mastery is generally not valued on tanks for offensive purposes despite granting 1% AP per point because so much power comes from external sources. Additionally, the reason so many tanks play DPS trinkets is because tank trinket design is generally underwhelming. In the rare cases where a tank trinket is designed to have a good enough effect that we would want to use it, they generally end up being powerful enough that DPS and healers want to use them - some examples are
Carapace psychique rémanente from Battle for Azeroth, as well as
Insigne des flammes,
Ragecœur souillé de Fyrakka, and now
Plaque d’effroi fortifiée in Dragonflight. In some cases, when the damage is high enough, the effect is reduced by an order of magnitude, making the trinket then worthless for everyone.
Most importantly, it's not fun for tank specs to lose a large portion of damage from these external effects, since it allows for multiple options - those who want to optimize damage can use damage trinkets, while those who don't care about it can use the defensive options. By nerfing the damage options, this just forces us to lean in the direction of using tank trinkets, but without a similar effect on tank trinkets for non-tank specs, this means that we're effectively fighting over a smaller pool of trinkets.
