I have a different answer. When G-d set about destroying the evil cities of
Sodom and Gommorah, Abraham argued with Him. He argeued and G-d agreed not
to destroy the cities if there were 200 righteous people, 100, 50, and so
on down to ten. G-d would save the cities if there were ten good people.
Abraham stopped pleading and accepted that number. Hence, ten makes a
quorum or a minyan.
Irene Stern Friedman
http://www.geocities.com/Heartland/5013