Strength is not the only basis for a Kage...but for wisdom, experience, strength and (....mention other other qualifications here...)...
You know hokage isn't mainly about strength or at least even if it seems they are stupid enough to make mostly the strongest ninjas the kage (tsunade) but still they wouldn't let someone like Naruto become kage cause inexperienced, hot-headed, not-to-clever, genin, and cause he would disobey orders which he doesn't like.
yeah but who else could they have chosen? Jiraiya didnt want the position... and i think the way the manga is going...Naruto looks likely!