Part of me says it's marketing with manufacturers asking themselves, "how we can make a bike look different, so people feel they have to upgrade" - like a form of planned obsolecence.. Seems like you can only do so much with a bicycle, yet capitalism is based on buying. Some technologies like disk brakes on a mountain bike fantastic. But going from a 10 spd to 11 spd cassette seems like product differentiation more than performance.
Compliance, shock absorption and comfort might be factors to curving the members, especially on road bikes with no suspension components .Also, perhaps hydroforming metal tubes has become common place and cost to form has really become low cost, making it available to mass production and soon to be found on x-mart bikes.
Good topic, gives us something to think about.