There's definitely more to the story here, IMO. There's a few situations that come to mind as to why only the father was charged.
Kleberg County Sheriff's Office jail records show Jerry Lee Martinez, the baby's grandfather, was arrested on charges of abandoning or endangering a child with criminal negligence. He is also charged with
tampering with physical evidence with the intent to impair and tampering with a witness.
Grandfather arrested after newborn found in Walmart trash bag
No hearing date has been set and no attorney of record is noted for the infant's grandfather. I wonder if his charges stem from him taking his teen daughter there with the intent to have her deliver in the restroom and abandon the baby?
and then there is THIS:
Mathis Police Chief Willie Figueroa told KRIS 6 News that
Mathis PD arrested Jerry Martinez's stepfather, 60-year-old Mario Duran. He has been charged with sexual assault and has been booked at the San Patricio County Jail.
His bond has been set at $300,000.
Grandfather of newborn baby found in Kingsville Walmart restroom appears in court, second arrest made
It is sounding to me like the teen who gave birth may have been SA'd by a family member and that is why her own father participated in this attempted coverup. I suspect the teen was coerced/forced into abandoning her baby by her father and step grandfather.