Because it’s all about popular 
Honestly, dangerous to house trans women in units designed specially for them on the male estate?! Where is the danger? Actual danger, I mean, like the actual violence & rapes resultant from transferring male persons - however they may identify - into the female estate. Not “violent male sex offenders who identify as women are not getting access to women and they are Dreadfully Upset about this”. Being upset =/= being persecuted, threatened, oppressed, or endangered.
Incidentally, does anyone know if trans women actually ARE at more risk in prison (in the male estate, for avoidance of all doubt) than gay men are? Nobody says that gay men are too vulnerable for imprisonment; nor are they offered protected spaces as a matter of policy; but they are vulnerable (& will be viewed by peers as transgressing all kinds of boundaries) in a prison environment, so… 