If you really need to check various kinds of vegetables for bugs, why is it that nobody ever heard of this requirement until recently?
I'll divide my answer chronologically and geographically.
1) In the Old World (Poland, Ukraine, Morocco) - Everyone spent lots of time checking for bugs. But they also spent lots of time grinding flour, plucking chickens, and a thousand other things which are now avoided thanks to modern economics and technology. So the bug-checking didn't stand out and you didn't hear about it too often. Also, salad-type vegetables were much rarer (nobody had refrigerators) and so it was less common to need to check.
2) USA before 1940s - Almost nobody kept kosher.
3) USA, 1940s-1970s - Pesticides removed the vast majority of bugs and there was no need to check.
4) USA, 1970s-present - DDT and other pesticides were banned. Also, international food importing allowed bug species from different parts of the world to spread to new habitats. The frequency of bugs increased from "almost never" to "as often as consumers are willing to tolerate". Suddenly, halachic issues emerged which had long been irrelevant and never before been the center of attention.
(Disclaimer: I have no special sources for any of this, but it all seems likely.)