Because they're sports cars, so have big engines and go fast... Basically anything bigger than about a 1 or 1.1 litre engine would be ridiculously expensive insurance for new drivers D:
I think it's cause new drivers are apparently terrible people and crash all the time, so if you crash a shiny fast car it's going to be worse (and more expensive to fix) than if you crash a banger with a top speed of 60...