Because it smells like hot dogs. And because your skin can't use that much Vitamin C. Ferulic acid is primarily added to ascorbic acid serums in attempt to extend the shelf life of a highly unstable serum. Unlike the popular (expensive) CE Ferulic Acid ascorbic acid serum, we do not have this problem––since our serums are bottled with out oxygen exposure and using an airless bottle. Ferulic acid also boosts the sun protective properties of ascorbic acid––but nowhere close to what you can get by simply applying sunscreen (which you should do anyway!). As documented by ResearchGate, the most important component of topical ascorbic acid effectiveness is its pH (it must be below 3.5). This study also shows that at a proper pH (like the Protocol Vitamin C Superserum at 3.4), the skin reaches a saturation point where it cannot take on more Vitamin C after just a few days of application. Based on these recommendations, we have lowered the concentration of our serum to 10% ascorbic acid, which allows for total antioxidant saturation but with a gentler daily application. In other words, 20% is more harsh and goes to waste anyway after day 3 of use.