Originally Posted by
daoswald
In a velodrome track you'll see single speed bikes with steep gearing, but you won't see hills, and the whole race is over pretty quickly. Gear changes would slow them down.
Track bikes have one gear because you're not allowed to use anything other than a brakeless fixie with one gear.
Multi-speed would be a significant advantage in many events, and some of the shorter events are actually where this is most true, since they have to make slow or standing starts: gear selection is a big compromise between acceleration and top-end stride.