Why did Chevy stop making small block 400 engine?

Chevrolet stopped producing the small-block 400 cubic inch engine primarily due to emissions regulations and fuel economy standards that became increasingly stringent in the 1970s. The larger displacement engine, while powerful, was less fuel-efficient than smaller engines, putting it at a disadvantage in the face of these regulations. Furthermore, the added complexity and cost of adapting it to meet the new emissions requirements likely made it less economically viable compared to smaller, more easily modified engines.