Without payment there is no easy way to attract more resources into the system.
Popularity is only one reason to store data.
Some data may not be popular but may have high value for a small group of people or have long-term archival value. I guess Freenet focuses on popularity because it's harder to manipulate than subjective value, but the downside is that it can't handle the latter kind of data. A market-based system can automatically and optimally balance between the two.