Ask HH: Why big companies with very smart engineers stop innovating?

I wonder why companies when then become big stop innovating. All these managers read Innovator Dilemma but still they stop innovating.

For example, we all know that Google has very very smart engineers. So I wonder why Google stop innovating?

The last disruptive innovation by Google was Gmail (I think).

Android is copy of iPhone (not Google innovation), Google+ is essentially copy Facebook, Google App Engine is answer on AWS/Heroku, etc.

I have been reading Innovator Dilemma and other books by Christensen but I would like to understand a little more what are dynamics inside big companies preventing disruptive innovations.

I would like to start discussion and inside stories from big companies about their attempts and failures to start disruptive innovations.