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.
Some sweeteners like Sorbitol or Splenda have calories. They get away with labeling in the US because as long as it has less than 5 kcal per serving they can say 0 kcal. And they can choose the serving size.
Most sweeteners are truly zero calories and don't produce an insulin spike, but there may be a problem with cravings for some people.
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.