Why didn't the Titans wipe out humanity yet? It seems like the Titans have a really huge advantage like the colossal, armored, female Titan. They could probably charge at Wall Sina instantly, take it down and wipe out humanity. The female Titan spared Armin's life a few times, when she could take him out in seconds.  

So it seems the Titans are capable of killing all the humans completely but they don't. Why is that so?
This has no answer yet, and probably will not for a while. The author has stated the publication will run for at least another three years from the date of the linked article (September 2014).

Below is what has been revealed so far: 



   The reason seems to be that the Coordinate, which has the power to control Titans, is "lost".  Mindless titans can't be controlled without it.


   It could be said that Colossal Titan only emerged recently to take down the wall since a Titan of that size and power has never been available before. 


   Titans are actually used to enslave/control the population (since the royal family seemed to have had the Coordinate power for several generations). They never attempted to use it against the "enemy" before Eren took control of it, which is odd. Bertolt, Reiner, Annie could actually be working for the king, since the royal family "lost" the Coordinate to Eren's father.