Why do track cyclers start slow?
They don't start slow because they are on a fixed gear (track) bike. They start slow because they are trying to coax the other rider into starting the sprint for the finish line before they do. The advantage is typically given to the rider behind the other because you have not only the element of surprise, but you also get a draft off the person in front.

These riders walk the line of not starting their sprint too early and thusly fading at the end...and allowing the other rider to get the jump on them and losing the advantage of getting in their draft.