Two main factors: Harolds army had already fought off one invasion at Stamford Bridge that year, were tired and had largely started to go home to get on with all the farming. On the day the English held the high ground. They were so keen to get at the enemy that when the first few waves of Normans were beaten back they charged down the hill, lost the advantage and all discipline, and were hacked apart. That plus the lack of a leader (Harold was killed) did for them.
The first invasion was Norsemen I think, with whom Harold's brother had sided in the hope of getting his hands on the rather unstable English crown. Imagine Wills and Harry getting up to that sort of thing in ten years time....