Originally Posted by
wannabepilot1991
Now i don't know why this limitation has been set, may be altitude cannot be maintained with reference to horizon only.
More likely because, for whatever reason, ATC does not want to deal with VFR flights in amongst all of the airline traffic. In Australia you can be VFR up to FL245. Rules about when you can and can't fly VFR have nothing to do with when you should log instrument flight time.