gandalf and the name incanus

[Holbytlass]

Originally Posted by Eomer of the Rohirrim

There might have been millions of willing fighters, untouched by evil, leagues and leagues into the east.

True, but Gandalf's purpose and main work was with Gondorian/Rohirrim/Shire lands. Since Gandalf travelled in regular sort of ways, not disappearing-reappearing in an instant he was limited in his scope, even with Shadowfax.
It would have done no good for him to leave his responsibilities and take over someone else's even if they came to fail.