If Bethesda can't perform a simple check to see if another DLC is installed and then enable/disable an option based on that, we're all doomed.
The bottom line, IMO, is not that its difficult to do but rather, whether Bethedsa consider it worth doing or not.
This "simple" check involves factors like load order and the check itself can result in a error.
DLCs are like a closed world. The only know themself and the main game. A precise check for something outside of the world is no simple task.
Sure, they can import a seperate adoption system for Dragonborn but this would result in:
"Ok child i adopt you.... but sorry the DLC you live in does not support the homes from Hearthfire so you can only live in the "vanilla" homes.
The bright side of this. More than 2 adopted childs are now possible.