I wouldn't say they're aimed at veteran mappers, veteran mappers just win these constest because they make good maps.
A newbie mapping competition would be problematic for two reasons: First, drawing the line between newbies and veteran mappers would be difficult, I have no idea whether I am a veteran already or still a newbie, most likely something in between. Secondly, not allowing everyone who wants to join is unfair, I know I would hate it if there was a really cool contest, but I wasn't allowed to enter because I'm too good at mapping.
In my eyes, the important thing about mapping contests, at least at tf2maps, isn't winning, but learning things and becoming better at mapping. The veterans you see here were all noobs once, and it took years of practise to become the great mappers they are now. A newbie mapping contest would pretty much tell every new mapper that being bad is okay because you can still win, when they should be encouraged to work hard and become better at mapping, and when they're finally good enough to win a contest, it will be a lot more rewarding too.
Just my two cents.
and the chances of quality maps being produced is low (theoretically), so interest in such a competition would probably not be sustainable for very long...
Edit: this too