the reason there arent a lot of quests for the 1-8 level group is that characters dont stay in that range for very long so its hard to get a group together for them, especially if the quest is going to run multiple sessions over a period of time. Your 1-8 characters will no longer be in that level group by the second or third quest. Also, even if there is a good group when a quest gets posted, they're likely to be out of that range when the quest acctually happens (1-2 weeks later.)
another thing is, its hard to justify IC turning away higher levels. Turning away lower levels is easier to justify IC i.e. "Were looking for someone more capable for this job." or "This is too dangerous, you'll likely fail/die."
Basically, the only way to turn away higher levels (except where there may be a certain individual that may be turned away for IC reasons like past actions or such) is OOC. The Lieutenant of X army isnt going to say "Were looking for someone less capable for this job." or "This is too easy, we want someone that it will be harder for."
The first answer is the real answer, the second answer is me pontificating on some other related topics your post made me think of.
-TV
Edit: edited for typo
