Hello! I'm also from one of these low-preforming areas in England, a town above London. Honestly, I think there's a lot of factors at play, and I'll talk in reference to a lot of people I know IRL and how I've seen them follow education.
Firstly, I think it's important to note that their parents are often from poorly-educated backgrounds themselves, which makes it harder to see themselves getting a good education, going to a good university, and working/living a high-brow lifestyle. There's always a few outliers of course, but generally people don't have high hopes. Similarly to this, there's more manual labour jobs advertised here than, I suspect, there would be in the city- like car repair shops, builders, plumbers, etc. People don't care about education because all they need is a few Cs, if that, to get a job- which is pretty important for them as this areas has a fairly high poverty ratio.
I also think the schools themselves are a big impact on how well they do/don't do. My high school was (is) pretty divided between people who were going to do well vs people who didn't care, and there's definitely elitism within the years that cut off lower-preforming students. This ties in with setting as well IMO, as there were some students and friends of mine I never got to talk to in class time because they were always in lower sets, and the lower the set, the worse the teacher! Not to say that the teachers were good either; a lot of teachers in these areas are ones that want to move, ones that can't get better jobs, or ones that grew up here themselves. This kind of puts students into the position of thinking they'll be here forever, which isn't something anyone really wants.
Equally, there have been many very good teachers who can't handle the disruption in class and leave to teach somewhere better. We had a couple of very good maths teachers who left for a better school in the country, and one in Africa, because they wanted students who were keen to learn and active in the classroom. Punishments can often be negotiated, even if the teacher doesn't want them to be, and rarely stand up as a means of discipline One example of this I remember was last year, in Year 11, someone in my class was having his phone confiscated, went back and forth with the teacher for a good 10 minutes (of class time), and ultimately 'won' in having her give up and go back to teaching the class. More extreme examples of this would be fights and people bringing weaponry into the school only having it confiscated and an often unanswered phone-call home to their parents.
When I entered my high school, we were promised a new building to be finished by the time we were in Year 9, but the government cut out funding and the construction hadn't started until our final term in Year 11. This goes across the board really- lack of government funding has left lots of schools in my area deprived from necessary things, like wages to pay good teachers, equipment (we ran out of exercise books, working computers, and even chairs for people to sit on), and so on. It just wasn't the proper atmosphere for a lot of students to work in, and time was mostly spent complaining than it was trying to focus on education- which didn't really matter because the teachers weren't great anyway.
Similarly, going off from slang, lots of teachers here use it which has led to some people writing it into the essays and work. I don't think there's anything to say students talking in slang can't write good essays, but it definitely goes impact. Notably, this area has a very low literacy rate- lots of people can't write well and struggle to read, but that could be because of a mixture of poor primary/high school education, as well as because a lot of people who come into this area have just moved from a foreign country and don't speak English that well yet. They definitely catch up, but for immediate figures it does create the idea that absolutely no one here can speak fluent English- which isn't true at all.
However, the college I attend right now has incredibly low expectations. Generally the goal is to get a C, and they will tell you that., It's not about improving your knowledge and climbing the intellectual ladder, it's about doing X, Y, and Z and using these buzzwords will get you a passing grade. Not exactly a technique I agree with, but it is teaching students that there's always leeways into passing grades, which I think gives them a lower impression of it. There's also a much larger proportion of students doing BTEC than there are of those doing A-Levels, which I think is simply reflective of how many students have been pushed away from traditional academics because of poor education beforehand,
Sorry for the rant! TL;DR: In my experience, it's the poor schools, parents history, teacher disinterest, and culture of the poor that pushes these students away from academic education. I'm sure I missed stuff but I just woke up and I always have a lot to say about this. Universities lowing entry requirements for students from low-preforming areas is definitely something I am completely for, and has helped lots of students leave this town and go to university to get a good education and eventually good careers! But I wish we had the resources and money to offer support for all students to help them reach their potential from the get-go. Lots of students are, eventually, left behind as failures while teachers put more effort into high-preforming students in order to maintain an ever-dropping rate of passing students.