Because fat and sugar are drugs. People don't usually think of them like drugs, because of the widely accepted, but very wrong, attitude that only illegal substances can be drugs. Sugar, fat, caffeine, alcohol or tobacco are all drugs. They all trigger a desirable chemical reaction in your brain, and all have addictive potential. At least 2 or 3 of them are also significantly more harmful to you than several types of actually illegal drugs.
And in general, though perhaps most strongly for drugs, many people suffer from the cognitive bias of "illegal=bad & legal=good. Automatically and by default"
He wasn't really a scapegoat, he is the one who caused it to tip over. Both corporations and citizens were polluting the lake, seemingly legally in the absence of regulation, or at least enforcement. After the critical state of the water was discovered by Lisa, everyone, including, Burns stopped dumping waste in the lake. It would have started to slowly recover over the next couple of months-years, as the newly introduced anti pollution regulations were (at least initially) effective.
Homer was the only one who actually illegally dumped waste, and he did so in full knowledge of the consequence, and for literally no other reason that he wanted to eat donuts after the health department shut down the donut shop.
He's not solely responsible for the pollution, but he is solely responsible for adding the straw that broke the camels back, and he did so in full knowledge of the severity of the consequences, and for utterly banal and selfish reasons.