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.
I used silica sand in a setup I had going, battled diatoms nonstop made the sand look really horrible, when I moved the fish, rocks, coral into a new tank i used arag, no more diatoms.
Aragonite is naturally developed and produced, and is said to contain properties for maintaining stable, ongoing level pH, on the slightly alkaline side just above 7.0.
The seabed floor around the Bahamas Islands, stretching out for hundreds of miles of reefs/reefline, is nothing but aragonite - hence it's popular use for marine aquariums.
I've used other sands. Some require WAY more rinsing or you have a cloudy tank. That's been the only difference I've noticed that was definitely caused by the sand. Otherwise, if the pH drops enough to break down aragonite, you have some serious problems. The only time I've had a problem with diatoms has been in tanks with too few cleaners--either because it's cycling, or the stock list means normal cleaners are seen as snacks.
Aragonite is naturally developed and produced, and is said to contain properties for maintaining stable, ongoing level pH, on the slightly alkaline side just above 7.0.
PH in a reef tank shouldnt really go below the high 7's, so it is a bit redundant to have sand that buffers it that high.
Remember, the water is buffered, the rock is basically where aragonite came from so it buffers and many tanks even do a constant buffering for the corals.