Because the candle heats the air, and hot air is much less dense (heavy) than cool air. The air is heated all around the flame but the hottest air moves upward rapidly. There is more; the chemical reaction (combustion) is less complete in the relatively cooler air near the bottom, but as that is drawn upward, combustion continues, releasing more heat as radicals like carbon monoxide and hydrogen oxide become the more stable forms of carbon dioxide and water vapor.
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