The six-phase model is a waterfall model. In other words, the phases take place in succession. Just as it is impossible to swim upstream against a waterfall, the pure waterfall method does not allow returning to a phase after it has been completed. During the implementation phase, it is not desirable to decide to adapt the design, thereby bringing implementation to a standstill. For a number of reasons (see e.g. McConnell, 1996; Kroll, 2004; Chromatic, 2003; Stapleton, 2002), the waterfall method is usually less suited to software-development projects.

Software development is a creative process.
It is nearly impossible to identify all of the requirements (functionalities) beforehand.
Estimating the amount of time that will be necessary to implement a functionality is quite difficult.
It should be possible for all intermediate results to be tested by users throughout the entire trajectory of the project.
Cyclical methods of project management
Conditions for cyclical project management
Risks of cyclical project management