I think it’s because left alone implies intention, and an explanation is only offered at the end of the headline. Whereas other phrasing can avoid that. Baby found alone after mother dies, for instance. Or even Baby found alone by police after mother dies.
Honestly, if I’m going to overthink it, I think it’s because of the cultural tropes of the U.S. and advertiser-driven media. Saying that the police rescue the baby sort of sets up the sentence for misinterpretation, but police rescuing the baby instead of merely finding it is more emotive - it drives engagement, it reinforces the notion that police are protectors.
And following, left alone vs found alone. Police rescue baby found alone […] is sort of narratively poor. There’s a disjoint that I’m sure someone smarter than me can describe, but Police rescue baby left alone […] is a better ‘fit’, even if it’s factually looser. It may have to do with cultural preconditioning where people expect police intervention only when the parent has taken an action.
Heck, Baby left alone after mother dies is saved by police, establishes the narrative without burying the lede, and it even keeps the left alone phrase intact while establishing context before moving to other narrative.
But anyway, my point, I guess, is that the title is editorialized for the wrong kind of drama, and that’s dumb. The situation has its own drama if they would appeal to empathy, rather than people’s desire to bootlick and see evil everywhere.