Because few people are all that panicked about lettuce, but everyone wants to know about pesticides.
You might not think that, if you read the aggressive marketing claims of organic food trade organizations like Environmental Working Group; they frame it for their clients that organic pesticides are somehow not chemicals. But of course they are. Copper sulfate is not only a chemical it is far more toxic than the more modern compound glyphosate (found in Roundup and others) it has to be used in much greater quantities. Though organic food in California is a tiny percentage of the market, their pesticide usage is 800 percent of its representation on store shelves.
In reality, pesticide levels on our food is safe, even if it's organic food and using more toxic chemicals than family farms growing traditional produce. The 2017 Pesticide Data Program (PDP) residue monitoring results found that "99 percent of sampled products had residues below EPA tolerances. The PDP data demonstrate that overall pesticide residues found on foods tested are at levels below the tolerances established by EPA and pose no safety concern. Based on the PDP data, consumers can feel confident about eating a diet that is rich in fresh fruits and vegetables."
What about that 1 percent? Organic farmers are allowed to spray the chemicals deemed worthy in their manufacturing process even the day they ship them to customers so they are as likely to be at higher than tolerances as any other food. And since organic food customers know little science, they don't understand that the reason you wash produce is bacteria, not pesticides, so they are more likely to eat something they pick out of a basket.
But I am not in the scaremongering business so I will assure you that even if your 1% of organic food has higher tolerance levels than EPA wants, those tolerance levels are often two orders of magnitude below the No Effect Level. So you would need a dose 100X as great as you are getting to have any biologically detectable difference.