Its mainly because it offers no buffering capability as aragonite (calcium based ) sand does. The beaches on the gulf side of Florida and such are basically all silica (quartz) based sands......I have used it for years in some setups and it does work ok, if you have a lot of live rock ., or just a FO tank.
The other thing I've read is that silica based sand has very sharp edges wheras aragonite is very smooth. If you have any fish that sift the sand, it will cut up their gills. It's also not good on snails or other critters that would move along it.