It's tough one isn't it. I agree that I don't think it is just about the relatively low pay. There are plenty of jobs which are badly paid.
I once worked for an animal charity. As well as the many volunteers working in the animal centres, the majority of the paid staff doing basic duties (cleaning kennels, mucking out etc) were on a pretty low wage. Every single one of them I ever met seemed absolutely dedicated to the 'cause', and put up with pretty horrible stuff (not sure I'd like to spend my day clearing up dog poo!), and did it because they adored working with animals. But of course - animals are cute. Adults with LD are not.
Think it's probably endemic in many of the 'caring' professions overall actually. It must be a top-down attitude. If top management cared more there would be rigorous recruitment policies and checks, ongoing rigorous supervision, proper training, and measures ensuring safeguards were in place. The fact that these people in this case were able to do what they did, so many of them, and probably for some time, means that supervision must have been minimal or non-existant. Or, if senior management did know, turned a blind eye. Not sure which is worse tbh 
It absolutely is symptomatic of society's view of adults with LD - also old people (hasn't similar abuse been uncovered in old peoples' homes?), and no doubt with elderly people with dementia or mental illness.
(Just as an side but related issue, my late MIL had long-term mental illness and other physical illnesses and for many years relied on homehelp to come to her house, make sure she'd eaten, taken her drugs etc. It was unbelievable how often the carers would drop in, leave food on a plate, not check if she ate it (she was malnourished as a result) or didn't take her drugs (including for life-threatening illness eg angina). DP made several formal complaints. The company was, of course, outsourced to a private company by the local authority.)
The fact is that some groups of people are deemed less important than others. So many factors perpetuate this.