The 2015 was the last of that body style, so it has the latest "guts" (CPU etc) with the previous keyboard and ports (USB-A, HDMI, Magsafe). Plus, with the non-mechanical touchpad, there's room for a slightly larger battery¹.

1: "The 2015 revision brought the modern Force Touch trackpad and used the space savings to increase the battery to 99.5 Wh, just under the 100 Wh carry-on limit for most commercial airlines" from Marco Arment, https://marco.org/2017/11/14/best-laptop-ever