The more expensive chips subsidize the cheaper ones. If they put ECC in low-end models, they would have to charge more for them, because fewer people would buy the high-end models.

Also, there's some cross contamination between price point and market segment here. Nobody just buys a CPU, they buy a CPU wrapped in a laptop. So Intel's real customers are laptop manufacturers, not you. So the low-end chips have to appeal to a model that the laptop vendors want to introduce. That takes the form of thin & light laptops (or low-energy-usage "green" desktops for office workers).

Adding ECC support adds heat and cost and die size. All things the thin & light market do not want under any circumstances.