A lot of mothers, especially single mothers, are so exhausted and stressed they're barely able to get through each day. Motherhood can be physically and mentally and emotionally shattering. To fit in hours of work each day on top of travel, housework, shopping, the mental and emotional load of parenting, etc. etc. leaves no time for things essential for mental health.
Of course, for some it's not so hard, because some have very different lives/bodies/experiences/situations/etc., so people like that perhaps don't understand that not everybody has their energy levels, their amount of sleep, their physical fitness, their mental health, their situation, etc., but for a very great many this is the case and they're already at the limit of what they can cope with.
On top of this, remember the Universal Credit system isn't supportive: it's intended to frighten people. It's based on threats, sanctions, and constant stressful meetings usually arranged at difficult or impossible times.
Add to that that childcare is difficult to find, work fitting school hours incredibly difficult to find (regulations ensuring employers had to fit school hours better would help), and many children are very distressed by long hours in school or childcare...plus the fact that many people are distressed not being able to spend time with their children (as working full time you only really see them for homework and bed, then have to spend the weekend trying to rest plus do all the chores that were meant to be done during the week)...
There are some reasons.
Also, many if not most jobs are very stressful indeed, so, added to the stress of parenting, can tip people over the edge.
Again, some people find jobs they enjoy or can at least bear with reasonable employers and have suitable childcare, but there are plenty who can't.
The anxiety and fear of being told you'll lose everything (money to survive, home, possibly your children if you're made homeless) due to not being able to meet Universal Credit criteria adds enormously to all of this.