Final answer:
Old textbooks are updated regularly to include current information and perspectives, making them relevant and useful for students and faculty. These updates, along with faculty feedback and the ability to customize content, contribute to the high demand and frequent unavailability of popular textbooks online.
Step-by-step explanation:
New editions of old textbooks are released frequently to reflect the latest changes and updates in various fields. These updates may include changes to economic realities, policies regarding international trade, taxation, insurance, and other topics that are subject to change over time. Modern textbooks are often revised to include newer perspectives, diverse contributions, and content that is relevant to student's lives and careers. These frequent updates are likely why it can be more challenging to find popular textbooks available online, as the demand from students depletes available supplies quickly.
Publishers also take feedback from faculty and incorporate the advice into newer editions to enhance the learning experience and maintain relevance. Additionally, textbooks might be remixed to match the specific needs of a course or instructor, offering flexibility and customization for educational institutions.