Because there is no standard for project planning data.

• There is no standard for project plans, and there is no standard for the data format of a project plan.
• Not all project plan apps provide the same functionality.
• Not all project plan data is the same in different applications.

MS Project is a well-known application.

• QuickPlan supports importing projects from MS Project XML file and exporting projects to MS Project XML file.
• Most other project plan apps support integration with MS Project via MPP file or XML file.

However, QuickPlan has no control over other apps.

• The XML file exported by QuickPlan is openable to MS Project App.
• QuickPlan has no way of knowing how other apps open an MS Project XML file.
• This is true for other apps as well - they can guarantee integration with MS Project, but it’s hard for an app to integrate with all other apps.
• A project plan XML file created by one app may or may not open correctly in another app.

QuickPlan is designed to support integration with MS Project by importing/exporting via the XML file.