The reason for that is given in stanza two where the poet explains that there was persistent drought and dams were dry and 'windmills stalled.' The most interesting thing about the description here is that she compares her mother's smile to the stalled windmill and the dry dam. One gets the impression that even though the mother was smiling it lacked the warmth of a genuine smile. We are also made to understand that such a smile almost always came when she had done something wrong. She continues to tell us that her mother smiling at their faults was a ploy to keep them from being chaotic or mischievous. What we learn from this is that the mother did not do anything to scold the children perhaps because she knew shouting and ranting was not going to work.
We are told that this situation was on-going. Added to all this was the fact the mother had to worry about how to provide the bare necessities of life for her children and ensure that there was food on the table. It was not a cheerful situation because 'each month was weeks too long.' This gives me the impression that the mother had to the hard work of ensuring whatever little money she had could be sufficient for the month but this was not the case. 'Her mouth laid clamped hard on this' shows us that the mother did not talk this