From what I can see from having a child with SEN, but not an expert, its a perfect storm of things.
There are small numbers of children with very complex needs who will have very high-cost support packages. This is a small number but probably larger than in the past due to medical advances.
A lack of investment in services - both special schools but also professionals like occupational and speech therapists - mean many councils are contracting to private providers at high costs because they are legally obliged to meet support packages.
Private equity firms are realising that this is a guaranteed area of spending so are investing in special schools that turn profits. This is criminal imo, but also the government aren't building these schools and children need to be educated.
Delays in accessing support and diagnosis, especially via the NHS but also within the school system, mean that some children are presenting with higher needs than they might have done, had they received support sooner.
Parents have come to understand that because everyone is so strapped for cash, time and resources, most schools/services can only meet the needs of children whose EHCP legally obliges them to do so. I've heard school speech therapists and educational psychologists say they are only allowed to work with children with EHCPs. So parents who might have once been supported within the school system and general budgets are now getting support packages, at higher cost. (To be clear, this is exactly what these parents should do.)
Schools in general are more academic, the curriculum is much larger, and there is more pressure on children to "succeed" - and there is a subsection of children whose needs make this environment impossible for them. There's less time for lessons these children might have thrived in - creative subjects, sport, learning outdoors - especially at secondary. Larger classes, and teachers who are having to spend more time on planning, data, record keeping etc. leaves less time for the kind of small group work and 1:1 attention that some children with SEN need.
On the other hand, schools are probably better at identifying children's needs. 40 years ago, a kid with ADHD might have spent a lot of time bunking off or getting into trouble, and failed all their exams, but no one would care because they might have got a (low skilled) job anyway. Or maybe the dyslexic child was directed to woodwork and PE and people just accepted they were failing English and anything where they had to write an essay. Now, schools notice and care. This is, again, a good thing but supporting these children comes with a cost.