Because the 15" Windows laptop screen size is the most popular, by far. It makes a lot of sense. 13" is a tad bit too small for students and office workers. 15

Because the 15" Windows laptop screen size is the most popular, by far. It makes a lot of sense. 13" is a tad bit too small for students and office workers. 15" is more comfortable for productivity. I assume that laptop buyers will focus on productivity. Otherwise, they'd just use their iPads/tablets for media consumption.