Because everyone says to use it... :evil_lol:
But in all seriousness, there are reasons. Silica-based sands can be used. They will contribute to diatoms, but that isn't the disaster people make it out too be. There are other algaes that are perfectly happy to fill in for the diatoms if you don't let them form, so the net algae levels remain the same. Aragonite looks a bit more nature, comes in a bit more variety, often doesn't need as much cleaning, potentially buffers the water (debatable on whether this matters) and doesn't contribute to diatoms (debatable on whether this matters). It also is less prone to scratching the glass then silica, although both will do it given the right circumstances.