why we can not create planner plans for private and shared channels

Hi johnjohn-Peter,

The inability to generate Planner plans for private and shared channels in Microsoft Teams is attributed to the integration framework between Microsoft Teams and Planner.

Essentially, when a plan is initiated in Planner, it relies on the presence of an Office 365 group. Since private and shared channels in Teams do not possess their individual Office 365 groups, the inclusion of a Planner tab becomes unfeasible for these channels.

There was a workaround to add a planner as a website in private channel, but that doesn't also work anymore.

You can give a feedback directly to Microsoft from your Microsoft Teams desktop app by clicking on ... > Help > Give Feedback

or you can post your feature request on the Microsoft Feedback Portal:
Microsoft Teams · Community