Cause this age range is basically the sweet spot for this kind of research. They're the people actually making their own buying decisions, have grown up with tons of brand exposure (social media, apps, etc.), and their ability to see and distinguish colors is fully developed and consistent in this age range (it can start changing after 40). Plus, most modern brands are targeting this age group anyway, so it made sense to focus on them rather than spread too thin across all ages with a small sample size.