for one, learning the basics might not be so bad on a 1000, but learning to do other things will be very hard, like learning how to ride hard in the curves or learning wheelies, you'll be too busy trying not to loop it and trying to hold back to actually learn, where a 600 you can really ride much closer to its limits and learn alot better

start two guys on bikes, one gets a R6, one gets a R1, give them both a year of riding and eventually move the R6 guy up to a R1, even though the R1 guy now has a year more experience on it, i bet the guy that learned on a R6 will be the better rider because the guy that learning on a R1 spent a year riding conservative and limiting himself where as the R6 guy was more free to ride the bike harder and learn alot more

hope this makes some sense, but other reasons are cost and insurance