Usability testing is often confused with market research, but the two are very different.
Usability problems can easily be detected from a sample of about five test participants. Very different kinds of people will tend to encounter the same usability problems. More than seven participants is just a waste of time.
Market research issues, things like design-preferences and whether they would actually want to use the product, are completely different. These vary wildly from person-to-person and different personalities/backgrounds. You need big sample sizes to judge them accurately.