I need help understanding why the Federal Reserve hasn't raised rates.

Ok, so the basic principle is that central banks such as the Federal Reserve or the Bank of England lower interest rates to stimulate the economy (when it's cheaper to borrow money, consumers and corporations are more willing to take on debt, e.g by buying houses or investing in production facilities, which in turn stimulates growth and increases employment rates). When growth is strong on the other hand, the central bank wants to ensure that wages and prices don't rise too fast, so starts to raise interest rates as a way to limit the risk of inflation. At the moment, the U.S and UK economies are emerging from a recession, with improvements in economic growth and employment, but these are still both relatively weak, so the central bank has to choose the timing and pace of interest rate increases carefully. If they raise rates too soon or too aggressively, they risk putting the brakes on a still fragile economy and falling back into recession. If they wait too long or increase rates too gradually, they risk the economy overheating, with the subsequent risk of inflation, or alternatively, enter another recession with rates already close to zero, having missed the opportunity to raise rates when growth was relatively strong and find themselves in the situation where they no longer have the main tool they can use to stimulate the economy. That's why it's important that they get the timing and rate of increase right, based on a range of economic indicators. Hope that helps.