The main reasons why Alibaba chose to outsource warehouse operations are as follows:

1. Cost-effectiveness: Outsourcing can help companies reduce costs. By outsourcing non-core businesses to professional third-party companies, companies can reduce their own operating costs and management costs. This cost-effectiveness is one of the important reasons why many companies choose to outsource.

2. Flexibility and scalability: Outsourcing can provide greater flexibility and scalability. Companies can quickly adjust the scale and scope of outsourced services according to changes in business needs without having to invest a lot of resources to expand themselves.

3. Professional skills and experience: Outsourcing companies usually have professional skills and rich experience and can provide high-quality services. Through outsourcing, companies can obtain these professional skills and experience, thereby improving their own operating efficiency and service quality.

4. Risk transfer: Outsourcing can help companies transfer some risks. By outsourcing certain businesses to third-party companies, companies can also transfer the risks of these businesses to the outsourcing companies, thereby reducing their own risk exposure.

5. Focus on core business: By outsourcing non-core business, enterprises can focus more resources and energy on core business, thereby improving core competitiveness.

6. Technology and innovation: Outsourcing companies usually continuously update and upgrade their technology and services. Enterprises can obtain these latest technologies and innovations through outsourcing, thereby improving their own competitiveness.

7. Human resource management: Outsourcing can help enterprises better manage human resources. Through outsourcing, enterprises can transfer some responsibilities of human resource management to outsourcing companies, thereby reducing their own management burden.

8. Respond to market changes: In a rapidly changing market environment, outsourcing can help enterprises adapt to market changes more quickly. Enterprises can flexibly adjust their business strategies through outsourcing to cope with the constant changes in the market.

These reasons together prompted Alibaba to choose to outsource warehouse operations to improve efficiency, reduce costs, enhance competitiveness and better respond to market changes.