Though shuttle combustion experiments began in 1985, the 100 to 200 small fires planned for a single shuttle mission are the most ever. The answer was the safety issue. It's taken a long time to get the hardware, but that means they took the time to do it right. NASA required that three transparent enclosures separate the small fires from the astronauts. Without the influence of gravity, fires in space burn without convection.
Convection is the upward flow of air as it is warmed by the heat of the blaze.
As the warm air rises, new supplies of oxygen are pulled into the combustion. Without the buoyant flows, fires in space burn more uniformly, forming a spherical shape. The fires planned aboard Columbia will range in size from a half an inch to 5 inches in diameter and consume a dozen different fuels. Those include small amounts of hydrogen and mixtures of alcohol and petroleum. The combustion experiments are four of the 33 planned during Columbia's flight.