Of your five senses, the ability to smell is the oldest one, from an evolutionary standpoint. It dates way back to how even the most basic living organisms can detect chemicals in their environment.
Other senses, such as sight and hearing, evolved much later. They also use a different pathway to the brain, taking information from your eyes or ears to the thalamus in the brain. The thalamus then decides where to send the signals next— it's like a sorting machine at the post office. Smell, however, has a direct line to the brain and bypasses the thalamus, heading instead to that spot near the memory-making part of your brain.
This is likely why smells can trigger such strong memories, along with the raw emotions that went with the experience. Senses that pass through the thalamus are easier to describe, but smell is much harder to talk about. For example, you can describe what chocolate looks like by talking about its color, shape, and texture, but you can only describe its smell by saying it smells like chocolate. This is probably why scents can trigger powerful emotional memories: There are no words to get in the way, only the "pure" memory and the feelings associated with it.