Because it's being defined in the bill in a way that makes trans people not exist. It is explicitly defining according to sex assigned at birth and nothing else.
Meaning it would overrule any form of trans legal protections and anti-discrimination law, since it would make trans people not be a defined group.
Like I said, because the change they're pushing for is to have sex and gender be legally identical and define it solely by assigned at birth and nothing else.
Which would legally erase trans people from existence and undermine any form of legal protections for trans people.