The answer of course is fear.
Once someone does something that challenges a strong belief system, it puts some into murkier waters that they cannot abide to live in. They would rather eradicate that difference than face the reality that things are not quite as they believed.
Societies today face great pressures to accommodate racial, political and religious differences which were not as evident when countries were more monolithic in structure. With global movement of people and ideas, we are being forced to live next to people whose dogma may be anathema to us.
This brave new world has in turn created factions which are fighting back against what they deem to be an unwelcome intrusion into their previously comfortable lives. This infringement will be fought against at all costs because people who don't look or behave like them will not be welcomed or accommodated. Transgender people provide such a challenge to others, but we are not the only ones. The intrusion into a public space of jarring differences, forces a discourse that others preferred were best swept under a rug and left there.
Of course, it is too late to go back into the genie bottle and so the world will be left to adjust with awkward political and social justice battles rattling the core of most societies.
Such is the price of change and of achieving tolerance.