well it's not because the car is brand new, not entirely at least...there are many things that factor in...
1) age of insured
2) driving record of insured
3) type of car
4) engine size and specifications
5) vehicle's safety rating
6) color (thats right a bright yellow car costs less per month than a red or black colored vehicle as it is easier to see)
now a perfect case for the age of vehicle question is if i bought a 1996 dodge viper in 1996 and it cost me $500 per month then, it would still today cost me $500 (if not more because of rate hikes and so on) because the car is still in fact a dodge viper...
and as for the sports car question, it's all done on facts, more sports cars and suv's cause accidents, and more young kids and crazy guys in their mid 20's - early 30's cause accidents in sports cars than any other demographic next to un insured immigrants...therefore it cost more to insure as you are that much more likely to be in an accident...
and as for young males, they are involved in more accidents than anyone else...so again, based on facts, it cost more to insure...
i know insurance seems like a bunch of crap, and im not a talking head here, but once you understand insurance, it all makes sense for the most part...its a tough business...every company loses so much money every day...if they didn't charge so much they'd be out of business and everyone would be b*tching because no one would be insuring them...its a darned if you do darned if you don't type of thing...