Depends who you ask.
Some say: Because it's usually silica based and will cause algae.
Others say: No, it won't cause algae
Some say: It doesn't have enough surface area for bacteria and other sand bed creatures.
Others say: It does too have enough surface area and sand bed critters live just fine in it.
Some say: It's full of poisons and toxins and pollution because it comes from the ocean.
Others say: Yeah, so does most everything we put in the tank and I don't see any 3 eyed tangs.
Some say: It's ugly and unnatural.
Others say: Most coral reefs don't even have much sand and it looks fine.
Some say: Sand is a timebomb, go DSB.
Others say: Go with whatever you like the looks of.
Anyway, there's no huge reason to not use sand from the beach, give it a good rinse and give it a try if you like the looks of it. But it's not like you're getting free live sand. Consider it dead sand.