Because Bethesda Softworks is pulling a Nintendo.
The Windows version of Fallout 4 launched on November 9, 2015, and fans was quick to notice that version doesn't support Steam Trading Cards.
Bethesda Softworks hasn't mentioned if they'll add Steam Trading Cards to Fallout 4 in the near future, but I wouldn't suggest holding your breath.
Steam launched their Trading Cards service in the summer of 2013, and fans have since begged Bethesda Softworks to patch Fallout 3 and Fallout: New Vegas to support that service. Fast forward to the autumn of 2015, and they have yet to fulfill that request.
Oddly enough, Bethesda Softworks was quick to add Steam Trading Cards support to The Elder Scrolls V: Skyrim.
Predict the future: Are you expecting Bethesda Softworks to add Steam Trading Cards support to Fallout 4? If you, when will that patch be released?