Well, we don't always have an Ultimate skin every year.
I mean, we got them in 2012, 2013, 2015 and 2016 after all. So we missed one for 2014, and now 2017. And if you want to get technincal you could say they missed them for 2009-2011 when the game first started XD but eh, they didn't even have those thoughts then probably.
It's likely they try to do them every year as they once said in their little graph post about skin percentages, that an ultimate was a 1-4% a year. Which is just ONE skin out of the other types they make that had far higher numbers. Though, that doesn't mean that there's 100% chance they are going to have one every single year.