Because trans people are way more likely to be the victims of prison rape than cis people.

In its Policy Review and Development Guide: LGBTI Persons in Custodial Settings, the National Institute of Corrections reported that incarcerated transgender people are 13 times more likely than their cisgender peers to experience sexual assault, making up 59 percent of sexual assault cases in prisons and having the highest reports of multiple trauma.

Furthermore, trans people are more likely to be incarcerated than cis people. 16 percent of trans adults have been incarcerated, compared to 2.7 percent of cis adults.

https://www.vera.org/news/gender-and-justice-in-america/transgender-people-at-higher-risk-for-justice-system-involvement

In any case, caring about trans victims of prison rape obviously does not disqualify you from caring about every other victim of prison rape.