Because there are enough sugar replacements that will spike your insulin regardless, and that makes you stop burning fat. Try entering Ketosis and then have something containing Sorbitol & Xylitol, just 2 pieces of chewing gum messed me up for a good couple hours.

This is not to say that there aren't better sweeteners than sugar, but anything saying "less sugar" or "zero sugar" is likely just a technically correct marketing ploy.