I think Alienware is just responding to market trends. Note that both them and Aorus have launched 15 inch laptops now, I think they figured that between 13/14 and 17 they still need some middle ground (which I really approve cos for the past 1-2 years it seemed no one wanted to do 15-inch anymore and now it made a comeback)
I dont really think SLI is becoming more popular. The big trend in gaming laptops now is slimness, especially now with the 980M being the first flagship GPU that can fit in a slim chassis, slim laptops can now have performance parity with traditionally bigger ones. SLI takes away optimus and some games dont benefit as much from it so theres a bunch of tradeoffs that some buyers may not be willing to live with.