Because you do nothing. That's right. The number one cause is neglect. To many people, a concrete floor appears to be dry, solid, and impermeable. But concrete more closely resembles a very hard, almost dry sponge. All concrete starts out wet (if it wasn't wet, you couldn't pour it). It can take years to dry completely. And even then, though you may not see water, more than likely there is water vapor in the concrete. And like a sponge, it can absorb moisture from the ground underneath the concrete and even from the water vapor in the air itself. One of the most beautiful advantages of concrete floor polishing, other than the beautiful, marble-like sheen, is that the maintenance of the floor is minimal. But that does not mean you can neglect it.The polished floor pictured here was neglected . . . no cleaning, no sweeping, no nothing. Even though the floor is hard and permanent, it's still a dry sponge. It absorbed moisture from the air in an amount sufficient enough to leave behind soluble salts on the concrete surface when the concrete dried. In the concrete world, this is known as efflorescence. It's a white, dusty calcium based scale that leached to the surface of the concrete as the concrete dried. The good news is that it will eventually go away on its own . . . if you wait about 15 years. If you don't want to wait that long, then do the recommended maintenance to keep the floor looking good. Sweeping, cleaning, mopping, emulsifiers . . . these are inexpensive and easy steps to double the life of your polished floor, and keep a clean, bright sheen to it.