That's because they were busy developing Source Engine 2 since 2011.
Rumors are Left 4 Dead 3 and Half-Life 3 will be released for sale on Source Engine 2 either before 2016 ends or who knows when.
Dota 2 has been ported to Source Engine 2.
Either way it still stands I want to see Bethesda Game Studios support Fallout 4 until 2018 and every new video game developed by Bethesda Game Studios to have at least three years worth of support.
In my topic about Todd Howard he said Fallout 4 will get support in 2017 as well.
So we will most definitely get more Far Harbor sized expansion packs sold in 2017.
Not counting Dragonborn releasing for sale in 2012 for the Xbox 360, ZeniMax Media Inc., Bethesda Softworks, and Bethesda Game Studios released Dragonborn for sale in February5th, and February 12th, 2013 and fully stopped support in April 2013.
That was up to one year and five months of support, but that was because of the hardware limitations of the PlayStation 3 (PS3) and Xbox 360. Hopefully.