why the maintenance cost is roughly 80% of the development?

Answer 1

The 80-20 rule has been used for a long time. It is a rule of thumb and as such is only statistically true. Software is created to automate tasks and as the labor in completing a task is reduced new aspects of completing the task become the portion of the task that requires the most labor. In addition operating systems and other associated software change. Those changes often require maintenance changes but always require somebody know how to make the changes. Automation of a task usually needs to be done within a few month's of having identified the need for automation. Another way of looking at the development time is that it should be very short compared to the time that the application will be used. An application that needs to continue being usable needs to have somebody familiar with how to maintain the application. The labor required to retain knowledge about how to maintain an application continues for all the years that an application is used.