The mother was/is in a mental hospital.
Plenty of details here:
http://www.joplinglobe.com/local/x1415465705/Police-Toddler-found-dead-had-been-left-at-home-alone
Note the comments by the grandmother and other relatives.
If mom was mandated to a psych hospital and during the morning workup it was discovered that she had a small child at home, who may have been at risk, the hosptial social worker would have called the police to investigate and get the local Child Protective Services involved. Therefore no call.