Because you wanted the closest available to the original PC display standard that programs are written for of 96 PPI. Figure using the standard 96 PPI (aka DPI) for windows based PCs a 1920 x 1080 monitor would need to be 20 x11.25 inches 1920/96 = 20 and 1080/96 = 11.25) and by using the Pythagorean theorem (a squared + b squared = c squared) that would mean a c dimension of approx 23 inches -- so the most accurate reproduction of the 1920x1080 resolution on a PC would be gained by going with a 23" diagonal monitor !