Why has the gbp to usd exchange rate dropped?

I am studying in England and also working, so i am getting money salary in pounds. Time to time i am coming back to US to support my family with my wage and after 1.5 years i'll be back to US. I've started to think what i'm gonna do with my earned pounds and looking the best way to exchange them to dollars. But i don't understand why has the gbp to usd exchange rate dropped so much?