You can find the answer to this in Chapter 1 when Kino and Juana take their son with them to seek medical attention when he is stung by a scorpion. What is key to realise is how Steinbeck uses the differences in the appearance of the doctor and Kino and Juana to reinforce the sense of social injustice that pervades the novel. Let us remember that, when Kino reaches the door of the doctor's house, he hesitates, remembering that the doctor was not of his race:
This doctor was of a race which for nearly four hundred years had beaten and starved and robbed and despised Kino's race, and frightened it too, so that the indigene came humbly to the door.
This helps explains Kino's reluctance but also his absolute dependence on the doctor. Let us consider how they are described however. As they begin the procession to the town to see the doctor, Kino and Juana are described by those that see them:
And the newcomers, particularly the beggars from the front of the church who were great experts in financial analysis, looked quickly at Juana's old blue skirt, saw the tears in her shawl, appraised the green ribbon on her braids, read the age of Kino's blanket and the thousand washings of his clothes, and set them down as poverty people and went along to see what kind of drama might develop.
Note how the emphasis is on the poverty and suffering that Kino and Juana have experienced, with the "thousand washings" and the "tears in her shawl." Contrast this then with the presentation of the doctor:
He had on his dressing gown of red watered silk that had come from Paris, a little tight over the chest now if it was buttoned. On his lap was a silver tray with a silver chocolate pot and a tiny cup of eggshell china, so delicate that it looked silly when he lifted it with his big hand...
Clearly, with the quality and material of the red gown highlighted and the physical possessions that are emphasized, the doctor is from a very different background, as as Kino fears, abuses his power by refusing to help them because they cannot pay.