Because it’s terrible optics given the context. It’s like when progressives freaked out when people misgendered that trans school shooter
Misgendering anyone is wrong, no matter what they did (and there's also the fact that it would literally be inaccurate reporting to misgender). Like come on, no one is suggesting that the trans shooter or the undocumented murderer be let go. They should both be in prison for the rest of their lives. But the only reason to deliberately misgender the shooter or call the undocumented murderer an "illegal" is because you want a socially acceptable excuse to vilify the groups that they are a part of.
I don't want to risk starting a trans debate, but I want to say you've actually got it backwards on that school shooting story. All the talk about "gendering" was what led to inconsistent and misleading reporting. The most notable thing about that story wasn't even the transgender component, it was that for possibly the first time in American history a Columbine or Sandy Hook-esque school shooter wasn't a young man.
