For the same reason any other project fails wrong requirements, wrong expectations, wrong outcome. No time to re-use and learn from lessons learned.

Worse case scenarios is your project plan just not detailed enough. Below is an example of failed project attempt.

• Sample software development project
• Initiation phase
• Project activities
• Project plan approved
• Manage communications planning
• Start project
• Manage development planning
• Create development plan
• Create resource assignments
• Migrate data
• Distribute information
• Manage quality
• Configure production environment
• Monitor product quality
• Audit code commentary practices
• Perform maintenance
• Identify procurement requirements
• Procure materials
• Monitor user acceptance
• Create product quality assessment
• Create summary statistics for historical reference
• End project

How to do fix this, put an experience project manager in place to build out a realistic attainable project outline with achievable goals.