If someone was asked this question I believe her answer should be “because I myself am committed to the “Kollel life. Because I believe in putting aside materialism in favor of spiritualism and I am willing to do with less or without to support my beliefs.”
In addition she should be asked additional questions such as:
How long do think or expect your husband to learn full time?
Who will support your family while he is learning? Have you actually sat down and figured out what a basic no frills or low maintenance budget is? And what is your back-up plan if the finances doesn’t come through?