AWS is a preferred choice for application development in the fintech sector, including stock trading app development, due to its robust, secure, and scalable infrastructure. Its wide range of services can support real-time data processing, secure transactions, and high-volume data storage, all of which are crucial for a stock market app. Moreover, AWS's pay-as-you-go model allows fintech startups to keep their infrastructure costs low while they scale, making it an ideal choice for trading application development.