I was hoping to delete my gmail account and have everything with Apple.
When logged into the Apple 'Manage my Apple ID' page, I see it's not possible to change primary email to an apple domain.
Is there a reason for this?
No, it isn't. Opinions vary on why that is but one plausible reason is that if your primary email address was your iCloud address and you didn't have an alternate non-iCloud address or rescue address linked to your account, there wouldn't be a way for Apple to contact you if something happened to your account that prevented you from being able to access it.
Unable to use icloud email as primary account
