There are more children surviving medically complex pregnancies, births (including prematurity) and infancies that would once have ended up as infant mortality statistics.
I grew up with a relative "brain damaged at birth" 60+ years ago. He had better access to appropriate SEN education and services than families do now. He grew up in the family home, was bussed to school and day centres then ended up in an excellent, homely residential setting. Their budgets in the 80s and 90s were much better than they've been in the last couple of decades and staffing was more reliable. The constant in/ outs of temporary agency staffing started in the 2000s.
Today, hopefully his birth would have gone better with better procedures to prevent the situation escalating, but also better postnatal care could potentially have reduced damage and improved his level of function closer to mainstream levels. There were residents in his home (often with Downs Syndrome) that probably would have ended up in mainstream schooling today.
It's all what-iffery though. Since my son was diagnosed autistic, it's also not impossible that that was in the equation. There were traits there but that's hard to separate in the face of significant LDs and Global Development Delay.
There are also more children being born to older parents (older father theory applies to autistic DS1), and maternal health in pregnancy is becoming more complex which can affects on childrens physical and developmental health.
Either way, the numbers are increasing and provision is declining.
Meanwhile there is a core of neglectful, passive parenting that results in children mimicking some traits of children with additional needs. My TA friend was regularly beaten up and attacked by a huge 6yo (deferred a year) that was wildly and aggressively out of control through parenting. He'd been assessed but not met any diagnostic criteria. That situation was diverting diagnostic process time and classroom support from children with SENs.