Insurance providers are well aware of the risks associated with young drivers, which is why there are a limited number of under 25s temporary car insurance quotes out there. Anyone who falls into the age range of 17-25 is considered to be a high risk due to their lack of experience on the road.
Insurers will tend to view young drivers as being more careless and irresponsible on the road, and subsequently more likely to be involved in an accident and need to make a claim. Without any experience or no claims bonus, you can almost guarantee that your premiums will be higher that your older counterparts.
That is not to say that there are some insurers who will be able to offer you more affordable car insurance quotes for a temporary policy.