Because it is a confusing line up in terms of Screen Size. That is looking at it from a Screen Size Perspective. Now if you look at it from a Price point segmentation it makes perfect sense, but Apple historically has also have their product line with minimal screen size options. Screen size changes used to be a big deal, Apple stick to 13.3" Screen Macbook for the last decades and refuse to adopt the popular and cheaper 14", desperate using them in previous iBook era.
This is both a good thing and bad thing, with this mentality it means we will finally get lot more choices with screen size and variation. And more changes coming ahead. As with the rumours of yet another change in iPhone Screen Size and MacBook Pro 16".
Changes are good, but also uneasy, I guess I am getting old.