I think in the past, special schools were seen as a "hide anyone who isn't totally NT and just teach them how to live, don't bother with qualifications".
Obviously there were children for whom teaching them "how to function in the world" was best. And there were children who were bright, even very bright, who were let down by this system, but weren't given the choice.
In the 90s parents began to fight for the right to choose. I knew a few in various contexts who all won the right for their child to be educated in a mainstream education. For some it was the best thing, for some it was a disaster-but parents, having fought hard, often didn't want to admit this.
Now special schools are rarer and much harder to get into. My observation is that they've now become very much for the children of the parents who have the time, energy and knowledge to fight for them to get in. Parents who don't have that, often end up having a child sink in mainstream until they have no option but intervention or pulling them out.
Also because there aren't the spaces, there is a group of children for whom a special school would benefit, but there's no way they'll get in because there are always going to be worse children than them.
Another issue, which I was astounded to hear from a friend who has a child at a special school, is that children are continuously assessed. If they are deemed to have improved enough for mainstream they can be asked to leave.
Now, if you have a child waiting for a space, that may be great for your child. But just imagine you have one of these vulnerable children. You've fought for their place at the school which is suiting them. It's small and nurturing and they're learning skills at their level. They've friends and know their teachers.
Suddenly they're told that they're being moved to mainstream. They're (probably) going to be thrown into a big comprehensive, where most of the other children have been there at least a year, knowing no one, suddenly finding they're expected to do GCSE.
And, what's more, if they totally flop and clearly need the special school, there won't be a space to come back to.