There are strong incentives in higher income brackets for tax purposes and such. In the lower/minimum wage sphere, marriage can have the opposite effect. You can loose access to various government programs such as food stamps because suddenly your family income counts. I've heard that support for single parents and low earning single parents has been increased in recent decades, which has led to lower rates of marriage and subsequent erosions to the general fabric in that socioeconomic sphere.

So a few things the government does is favor oneside in divorce and family courts. They will give housing and money through programs to keep people single via section 8 and foodstamps programs. They incentivize single motherhood by allow fathers to be replaced by government checks.

All these things whether good or bad makes it less likely people settle for pairing together like they typically would have.

To be clear, I don't have a strong opinion about this, and I'm not making a claim that marriage is a good or a bad thing, I'm just answering your question.

The usual argument that says that the government disincentivized marriage is through social safety nets like welfare, particularly ones that pay more for single parents without a spouse in the house, as well as mechanisms like child support payments.

The usual argument that says society has disincentivized marriage is basically through access to sex. There used to be a culture of enforced monogamy, where there was enormous social pressure to be married before having sex, and especially before living with one another. Now, that stigma is largely gone, so the incentive to get married has diminished substantially.

Again, not making a value judgement, just answering the question.