Because they are handled by separate servers and separate teams.

They did share logins with addons.mozilla.org and forums.mozilla.org but it was not without problems as noted in https://forums.mozilla.org/viewtopic.php?f=26&t=26019

So it is not a simple thing to share logins on sites with different software used.

Some Mozilla sites do use Persona for logins. https://www.mozilla.org/en-US/persona/

Thanks for the replies. How unfortunate that Mozilla Support doesn't have continuity across to it's applications. Oh well.

Thanks again.