Why not a full gui? I think I can give a simple reason. Desktop Gui libraries are large and complicated. Assuming you want a full featured cross-platform the choice would probably be qt. But Qt suffers from a lot of api version churn(much faster then c++ standards), includes language extensions(moc) and does a lot of things in a non standard way for c++(including using its own collections and strings).
The alternative I can see would be to write a very minimal functionality cross-platfrom gui library or use something other library that might not as cross platform(lacking mobile platforms(recent initial qt support but likely to improve in the future), not as easy to port, not as good looking on some platforms)
