I get what you mean. I've suspected it's a combination of factors:

1. People have a name for it now. You can't announce or be prideful about something that you don't have a name for

2. People are more accepting of autism now. You'd be more incentivized to hide autism if people thought it was a bad thing

3. Autistic people tend to attract other autistic people. If you know one autistic person, you probably know a whole bunch of other autistic people too

But also, I just think that a lot of people underestimated how many people were autistic back then. A lot of high-functioning autistic people can pass for normal until you really get to know them. For instance, I'm like 99% sure that both of my parents are high-functioning autistic, and nobody ever suspected they might be. I brought up the possibility to them and their response was just, "yeah, I figured."