Why can’t I delete the primary Apple ID email alias or change it?

Basically because Apple says so: Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."

If you want to know why, you would have to ask them (but typically they do not provide reasoning for policies unless they already include it in their documentation). Click on this link for Apple products feedback links --> Product Feedback - Apple

Apple may not respond but your comment will be read.