I don't think there are more SEN kids - there are more SEN kids in mainstream because there's nowhere else to place them.
And - modern life is just way more difficult for kids that previously would have coped. Some kids, years ago, had 'odd' traits but largely got by - the kids that struggled in PE or stood at the edge of the playground. Kids with messy handwriting, disorganised, poor listeners. They have always been there.
But, in classrooms of old with forward facing desks and didactic teaching methods where you were drilled in maths, spelling, handwriting etc. That is going to suit an autistic mind. Kids did everything en masse. They sat, were quiet, copied what was on the board, went out to play together. Pretty regimented.
Even life was easier - if you were female your choice was largely to grow up, marry and have kids. Clever girls might have been nurses or teachers.
Boys - a trade, army, traditional male role.
And more mums were at home with their pre schoolers so there would not have been all the rush and stimuli that there is in today's homes. Mum would have been busy with the chores, and less severe SEN kids would just have been able to develop in their own time .
Of course they'd have had an awful time from their peers - and class teachers also could be pretty harsh on kids back in those days.
I still remember when i was in P1 back in 1971, our teacher standing over a boy and being very intimidating because he could not/would not drink his school milk through a straw and she shouted at him that "even babies can suck!"
He might well have been ASD as, some years later, a teacher told him to "pull his socks up!" (As in improve your work) and he bent down and pulled up his socks. We all laughed, including the teacher.
My memory of him is that he was always the teacher's target in every year and us kids just thought of him as a bit simple.
But that was one child in 30. The rest of us managed primary but we definitely had our share of anorexics, nervous breakdowns etc at high school.
So - always been there plus modern life IMO