Why does a wife who has no children not inherit her husband's entire estate, and what is the wisdom behind that, and how can she manage her affairs if she has no income or supporter?

If the deceased has no lineal descendant (child or grandchild), then the wife inherits only one-fourth, based on Allah’s Almighty statement: "But if you have no child, she [the wife] will have one fourth of what you leave" (An-Nisa: 12). If she is poor, then her maintenance is obligatory upon whoever among her heirs is wealthy, based on Allah’s Almighty statement: "And upon the heir is [responsibility] like that [of the father]" (Al-Baqarah: 233). If she has more than one wealthy heir, then her maintenance is incumbent upon them in proportion to their inheritance from her.