In the 1980's mountain bikes pretty well took over the market, and a lot of customers got reintroduced to cycling by getting a mountain bike, which they often rode on the roads. When those riders got interested in getting a road bike, they were used to wide handlebars and sloping top tubes, and suddenly these features began to be normal for road bikes. At this point it was discovered that older bikes have narrow handlebars, which was odd, because it hadn't previously been the case.