"for various reasons" ... not really, the main reason was "supposedly healthier." There was pressure from healthy food advocates, mostly based on pop science claims. In the 1980s the "fat and dietary cholesterol is bad" trend started. That's when restaurants switched away from beef tallow, so they could advertise "fried in vegetable oil" as if that was healthier. They also introduced salads, oat bran muffins, changed from milk to yogurt ice cream, and other things that customers generally didn't want but could be claimed to be "healthy."

The amount of sugar added to prepared food took a big jump around this time, as it replaced fat to make the food taste good. Around this time is when obesity started to become a bigger problem.