Several reasons.
First, if he copped a plea and she took it to trial, he's most likely to be the one that gets cut some slack. The system has a vested interest in discouraging trials. Particularly 2 week long trials.
Second, he's the one most likely to be able to support the family and keep it together while the other parent is 'away'.
Third, his plea was to furnishing alcohol to a minors and corruption of minors. Fairly low level charges. She's looking at time for two counts of involuntary manslaughter and endangering the welfare of a child. A quick google search indicates she could get up to 5 years on each of the manslaughter charges (It apparently is a first degree misdemeanor there).
Well maybe this is just me being stupid and lazy and not reading the article but how does the person who is not present and did not purchase the alcohol get charged with involuntary manslaughter while the person who DID purchase the alcohol and corrupt the minors does not get charged with that?