There may be several reasons why Douyin e-commerce doesn't sell big brands:

1. Brand authorization issues: Big brands usually have strict authorization policies, requiring e-commerce platforms to sell their products only through official channels. If Douyin e-commerce cannot obtain authorization from these big brands, it cannot sell their products.

2. Distribution channel conflicts: Big brands usually have their own official offline channels and online channels. If Douyin e-commerce is allowed to sell big brand products, it may compete with these channels and lead to distribution channel conflicts.

3. Brand image and positioning: Big brands usually focus on the shaping of brand image and positioning. They may think that the user group of the Douyin e-commerce platform does not match their target consumer group, and selling on this platform may have a negative impact on the brand image and positioning.

4. Brand protection: Big brands usually have high requirements for product quality and after-sales service. They may think that the Douyin e-commerce platform cannot provide sufficient brand protection measures and worry about product quality and after-sales service issues.

It should be noted that the above are only possible reasons, and the specific reasons may vary depending on factors such as brand and platform policy. The Douyin e-commerce platform is also constantly developing and improving, and may cooperate with major brands in the future.