Humanity has been around a long time. Why did Christ come only so recently?
St. Paul says that Christ came 'in the fullness of time'. There's a fair amount of Patristic ink spent on what that means, certainly more than can be covered in a message board post, but the short form is along the lines of 'human religious development'. God chose out Abraham and then throughout the Old Testament worked to bring them to a level that when Christ did come at least some of them would be prepared to accept His message and carry it to the rest of the world.
And by some of them, many Fathers specifically emphasize the Theotokos, the very best that humanity had to offer God as His mother.
(or in other words, even shorter form, Christ came when He did because that's when His mother lived--He may be unbounded by time and capable of Incarnating at any point in time, but as His mother was to be human, she was a specific person living in a specific time and place).
Why are so many of the symbols associated with the Theotokos also associated with various pagan gods? (I apologize for being vague about which pagan gods, I forgot which ones he mentioned)
Like Ozgeorge, I'd have to know what symbols to be able to respond because nothing in particular spring to mind.
I will say more generally, that the Church did not have a problem borrowing symbols and language from the cultures it encountered when doing so helped to communicate the Christian message--as one example, when the Church chose to set the Feast of the Nativity on the day of the Winter solstice. The Fathers were perfectly aware of the pagan significance of the date, and appropriated the day because that symbolism was compatible with the reality of what Christ's birth meant.