1. Economic factors: The salary man is slowly going away and so are the institutions that revolve around him such as mainstream Japan's definition of a family.
2. The younger generation wants more freedom from a patriarchal society; in particular women. On a whole younger Japanese women are not content with being trapped as a housewife. (It doesn't help that women older than 25 are derogatorily referred to as 'Christmas cake', something past its due date of 12/25 that no one wants.) Younger males are also no longer content with being trapped as a corporate slave who lives mainly for his corporation and functions mainly as an ATM for his family that he rarely sees.
3. Xenophobia: Even though the younger generation is more tolerant, on a whole Japan is a racist country. For example, unless you have Japanese ancestry it is near impossible to get Japanese citizenship. This has larger consequences such as really low immigration numbers and generally, non-existent integration of immigrants into mainstream society (yes there are always exceptions depending on which part of Japan we are talking about). In Japan's defense most of Asia is also racist, but Japan just takes it to another level.