Instinctually, Juana knows what is best for the baby. She does not need to waste time consulting with her husband, whose job it is to make a living, provide shelter and possessions, etc. (These differences are basically understood, but unstated.)Juana's job is to care for the family in a more subtle, feminine way. Later in the novel, Steinbeck again nods to the female's ability to understand things at an instinctive level when she pleads with Kino to give up the pearl. She understands the importance of ridding themselves of this evil possession and she even goes so far as to attempt to throw the pearl back into the ocean.

She's a mother!

"She looked up at him, her eyes as cold as the eyes of a lioness.  This was Juana's first baby - this was nearly everything there was in Juana's world."

As the father of a two year old, I've learned one very valuable thing: when it comes to the safety and protection of our little guy, there is no consulting with his mother.  I don't mind him jumping on the couch or off the second stair; his mother gets nervous when he walks across the living room.

Juana not only feels threatened for her only child's life at this point, she also knows how deadly scorpion stings can be.  For her to stop and try to reason with a stubborn male about the necessity of a doctor, could be the difference between Kino living or dying.  Frightened mother = quick decision.