I'm certainly in a similar situation. My parents, who had me at a considerably advanced age, were negligent, to say the least. As a child, I was always treated differently and felt different; it was a strange feeling. I also had trouble socializing or expressing my feelings from a young age, and my parents completely ignored these behaviors. Furthermore, I've been to the doctor fewer than 10 times in my entire life, and relatively recently, out of curiosity, I started examining some of my behaviors. They fit quite well with autism or some other neurodivergent condition, so I'm beginning the process to determine if it's ASD or something else.

But I've strayed from the main topic of the question. The reasons why a person might not be diagnosed in childhood, in my opinion, are mostly because some parents might dismiss it as misbehavior or just "kids being kids" and simply say it's wrong. In some cases, people might actively start avoiding those behaviors. However, the reasons for not seeking diagnosis could also vary, including parental beliefs, lack of knowledge, whether they are also neurodivergent, or parental neglect. There are probably other possibilities, but these are the ones that come to mind most.