My best guess is that for certain goods, people make purchasing decisions around major life events like getting married or becoming parents. If they have already crossed those thresholds those purchase patterns may be harder to unseat and replace, they may already be solving those new needs and have habits around existing brands.

My second guess is that DINKs have more disposable income.

Disposable income is Gross Income minus Taxes. A 2 parent family with 2 kids making the same as a DINK couple would have a lower tax burden and more disposable income, but the DINK couple almost certainly has more discretionary income, which money left after paying for necessities like housing, food, and medical care.