It's all about what is trendy. What part of the population should we optimize for now? Some people like big text and wants to scroll. Other people might have problems with their carpal tunnel and can't scroll with the scroll-wheel on the mouse. Mabe they don't even have a mouse at all and scrolling has to be done with arrow keys and a bad angle on the wrist? It's hard to optimize for everyone so usually you just listen to the people that has been testing different layouts already and measure on how many adviews you get to deside what layout to choose. Because at the end of the day that is all that matters to most. Where do I get most money?