Whether or not people believed in the extent of surveillance largely comes down to how cynical people were about the US government.

I remember having conversations with people all the time in the 80's and 90's who refused to believe it even when they were told about things that the US government openly admitted. A lot of people had a lot of trust in the US government. Partly I guess because before the fall of the Soviet Union, to a lot of people it was a black-and-white good vs evil thing. If the Soviet Union was evil, then the US had to be squeaky clean and flawless. It of course helped that most people had no access to the internet.

I'm not surprised it's taken this long, given how hard people try to hold on to their beliefs even in the face of evidence. The fact of the matter is that we had, and still have, very little evidence about this surveillance, which slows the process down even further.