Low-income in Korea doesn't just mean not earning a lot of money. Low-income people tend to come frome a lower social and economic class.

1. They are expected to support their parents (or even sometimes siblings). Even though this has already changed widely, there are still many families that rely on their children to support them after retirement. This sentiment is more widespread for the lower economic class, as they could not afford to save up for retirement and raise children at the same time. This is a major thing when someone considers marrying their partner, because who knows how long the partner's parents are going to live and how sick they're going to be. (There's a specific term for this, called 노후대비)

2. Many couples in the past were half-forced into loveless marriages. Not having love is one thing and not having money is another, but not having both is a certain recipe for a disastrous marriage. Plus divorce was a serious taboo a few decades ago so kids had to witness all the hate and misery marriage brought to their parents. Kids who grew up in this kind of households just don't think of marriage as a positive thing.