As others said, part of it is more awareness. Likely a significant part.
There are also theories as other said about rising age of parents, more and earlier premature babies surviving and other babies that wouldn't have survived before, a more complicated and sensory intense society causing developmental changes and making 'low level' issues harder to handle, substances in the environment from plastics & air pollution to drugs both old and new, as well as society getting more pathological and seeks out a medical explanation for things that often weren't before.
The awareness also brings about the theory that part of the rise is misdiagnosis. There is signficant overlap between neurodivergent conditions, and people can have more than one. Professionals, parents, and people seeking their own diagnosis are more likely to investigate conditions they hear more of and often once one label is found and, especially when socially acceptable to them, don't seek further unless there is significant further issues, even if another condition is more likely.
There has been the controversial argument that part of the rise is FASD (fetal alcohol spectrum disorder, now used instead of having multiple diagnosis about in-utero alcohol exposure). Not that women are drinking more while pregnant, but that it's being misdiagnosed through lack of awareness, social acceptability, and that either evidence or admission from the mother of drinking alcohol is required for diagnosis. FASD is co-morbid with other neurodivergent conditions alnog with physical ones. Research into the area has FASD as common to twice as common as autism, in some areas significantly higher, but it gets far less awareness and there are far fewer places in the UK able to diagnose especially for those without the facial features visible in 10-15% of FASD births. This can be an issue when the same symptoms in both present differently (a child who struggles with social cues and withdraws from it is very different from one who struggles with social cues and recklessly pushes for social interaction -- the latter is much more common and tends to presents younger or first in those with FASD before becoming socially withdrawn and this needs different support).
Trauma dysregulation can fall under neurodivergency, and there has been research discussion for a while about the overlapping symptoms particularly in ADHD and trauma disorders, but also with autism, both in trying to tease out how to distinguish them from each other and how to treat a child with multiple when different supports are shown to be more helpful in each type of issue.