A market research team at a video-streaming service wants to estimate the proportion of all current subscribers who would pay an additional $5 per month for a commercial-free plan. The team randomly selected 20,000 subscribers and emailed each one a questionnaire. A total of 1,900 subscribers completed the questionnaire, and 1,330 of them indicated that they would be willing to pay the extra $5. Based on these results, the team concluded that about 70% of all current subscribers would pay the additional fee.
Which of the following best explains why this conclusion may not accurately represent the sentiments of all current subscribers?