Markets hate political uncertainty of any sort. If you're a financial institution or a multinational corporation you want to know what sorts of proposed legislation you're likely to deal with, what tax rates are likely to look like and what monetary policy is going to be followed. When there is uncertainty about big items like that, the markets react. It's safe to say that continued political instability would have been very bad, and if Cameron is forced to form a minority government then the markets won't react positively even on this news.