Well, because law enforcement officials in the U.S.A aren't permitted to use overtly harsh interrogation techniques like torture.
That being said, I think the hope is that, by having charged them with child neglect and then proceeding very slowly through the legal process, a kind of war of attrition is underway. Neither Vang or Baur can make bail, both have made efforts to have charges reduced or dismissed (Vang) or bail changed to signature only (Baur) and those efforts were rejected by the Court.
They've been sitting in jail for four months, stewing in their own cesspools of BS, and maybe the lack of freedom - never mind her longing to find her little boy, because apparently she doesn't care about that - is finally starting to get to Baur in particular, if yesterday's suggestion by her public defender about a possible resolution to the case means what I think it could. I'm hoping she's ready to give up Vang in exchange for her own plea deal.
JMO, the patience of Two Rivers LE is amazing. They will do nothing to compromise their ongoing investigation and relentless searching. Unlike Baur and Vang, all Two Rivers LE want is to find Elijah, dead or alive, and return him to his extended family, the only people who truly did love him, it seems.