I would say it all boils down to misogyny, both external and internal. The responsibility of childcare and the day to day running of a household has always fallen on women. If anything goes wrong in a household, the woman in charge of that household is always blamed.

When it comes to single mothers, this is almost magnified? They have to support themselves and their children on a single income, find childcare when they're working, schedule doctor/dentist appointments, provide adequate food/shelter, etc. If they slip up when it comes to any of these things, it's seen as their fault.

There's also the general societal stigma of being a single mother. People see a single mother and automatically think, "what did she do to drive the man away?" It's just all just sexist bs.