First of all we must realize that it was the most important change suffered by oral English in history. Thanks to that big evolution in the language we have our modern English. The sound of large vowels was suffering subtle modifications along three hundred years. Linguistics didn't notice that until it had finished.As there is not a clear reason for this historic fact, some theories have been developed about this subject. The strongest hypothesis makes reference to the comparison between English and French pronunciation. As far as England had been a French colony for centuries, when colonization finished English people wanted clearly differentiate their language from French. Academics supporting this theory remark the similarities among the old English and the French vowels pronunciation.