I feel that unlike with ex-gays (who could use the bisexual or "exploring their sexuality" defense), detransitioners are seen poorly because they're living proof that transitioning isn't the end-all be-all point. The tranny's entire argument completely rests on the fact that transitioning will unlock the person's true self and that preventing transitioning is akin to performing medical malpractice, harming them, or putting their life at risk. The trans community outs the detransitioner and smears them (or claims they weren't transgender all along) to prevent the idea that transitioning isn't the ultimate goal.
The trans community wants to paint the top/bottom surgeries as being functionally identical to their genetical counterparts. This is not true. Detransitioners that detransitioned after they got bottom/top surgery are seen as special threats to the trans community, since they're more likely to frankly and openly talk about the medical process, the poor results of the surgery months or years after the operation, and how the medical team that treated them basically abandoned them once the detransitioning patient had questions about the poor functionality/healing.
Detransitioners also likely to talk about how they were rushed through the process and greenlit for hormones and surgery, instead of being given therapy in order to see if they were truly dysphoric or just autogynephilic/autoandrophilic. Such therapy is seen as a barrier to the trans pipeline, and is decried by the trans community. Detransitioners are also more likely to advocate against the use of hormones and puberty blockers on adolescents and children, as well as the internet forums/websites which convinced them they were trans in the first place (grooming), which the trans community in general is favorable to.
It is also why studies that discover that post-op trannies are less happier and more dysphoric than their pre-op cohorts are suppressed.