Couple of reasons: First, he was a man with a considerable ego. There is no reasonable argument against him needing to be the centre of attention and the top of the heap.
Second, the Revolution, capital r, had failed. After the execution of Robespierre, certain people in positions of influence in France began to realise that popular rule and Rousseau's idealism wasn't helping anyone, in the form that it had manifested.
Hope this helped!
