The way the prosecutor argued, using her leading a normal life during pregnancy against her, smacks of misogyny.
I am rather sure the sentence reflects that attitude. As AskBasil pointed out, childbirth leads to an exceptional psychological state, and it is not fair to treat the killing of a newborn like murder.
The accused herself says she thought the baby was dead. That may have been wishful thinking, but there's no sign of violence against the baby, so it seems to have honestly been what she believed.
Considering how easy police officers who kill black people get away in the US, and rapists, and ... well, literally every male criminal (who isn't black), it is obvious that this young woman is treated unfairly.
She should be punished, of course. But there's little danger of her repeating this; she is remorseful, and she is certainly not a danger to the general public.
What will putting her in prison actually achieve? Put the fear of the government into all those young women who are, in cold blood, planning to murder their newborns immediately after birth? Please. It is not like this is a common crime, and it certainly isn't a planned one.