The main reason is because it's implemented as vectors. Moving forward gives you a forward push, and moving sideways gives you a sideways push. Which means that the fastest way to move would naturally be by moving diagonally. This is what happened in AC1, where people literally run diagonally across the countryside. It looks ridiculous!
So originally sprinting was a special move that can only be done going forward in Gorgon. After lots of people asked for it, I added simple strafe-running, which is still a bit faster than walking, but the speeds don't sum to be faster than running forward. Which means running diagonally isn't a useful tactic.