Per my answer [here][1]; 

The Silence needed the suit to physically control River Song. They're aware of the events surrounding his death (e.g. **that he's shot by River Song at Lake Silencio on the date in question wearing an Apollo spacesuit**) and must have been quite surprised to find that she had no intention of doing so.

Their reaction was to travel *further* back into time and guide the development of the human race to the point that they'd developed a space suit capable of being used as a physical control device, turning her into a 'people puppet'.

In the episode 'The Impossible Astronaut' River Song is later reported to have said to the Doctor (at the moment of his apparent death): 

> "I can't stop it - **the suit's in control**".

As to why they didn't try something less convoluted (e.g. altering the entirety of human history just to get hold of a space suit that they could have easily replicated themselves) the answer is that it's not been explained.

  [1]: https://scifi.stackexchange.com/questions/46980/why-did-the-silence-mess-with-earth-history