Why did William win in the Battle of Hastings in 1066?

The best way to approach this question is to weigh both sides of the answer - was it due to William's strength or Harold's weakness?
The battle lasted the whole day – which could suggest that the armies were evenly matched at the beginning
English:-       Fought on foot-       Used their shield wall to withstand repeated and bloody assaults-       Housecarls w/ battle axes
Normans:-       Strong cavalry force of 2,000 knights and squires to support the infantry-       Normans gained upper hand due to skilled archers – able to break the shield wall