This is at least in part due to UC being so inflexible and punitive. It's very all-or-nothing - you're either able to work full time and hit the earnings thresholds, or you are deemed to have LCW/LCWRA and have zero work commitments. There's no in between for those who could manage part time work, but not full time, but don't want/need to claim disability benefits.
There's been a big fuss made about people who were self employed on tax credits but making very little money, and the solution was posited as UC and the minimum income floor.
But fact is, a huge number of long term unemployed (and let's be honest, often unemployable) people back in the 2000s/2010s were strongly encouraged to go self employed and claim tax credits. It suited the government of the time to get them off the unemployment figures. Many of them were never going to be capable of supporting themselves fully via self employment, but the old tax credits system allowed for that and they ticked along ok. They were poor, because they didn't earn much and tax credits only topped them up so much, but they managed to survive.
Also, old style housing benefit was not conditional. It was solely income based - if you couldn't afford a home, it helped you out. Now, UC housing element is subject to the same conditions as the rest of a UC award. If you don't meet your work commitments, you stand to lose your home.
Now that UC is pretty much fully rolled out, those chickens are coming home to roost. That same group of people are not magically employable or capable of full time work all of a sudden - they're now having to claim disability benefits in order to get the work commitments lifted or to prevent homelessness, whereas before they could manage on the subsistence income they got from part time work plus tax credits and/or housing benefit.