It's the mysteries of the insurance industry, innit.
Ultimately insurance premiums are based on a complex set of data which allow the underwriter to estimate for a given profile of driver and car how likely it is they'll have to pay out, either because of an accident or theft, and the average pay-out.
Over time this data can affect itself - so if, say, lots of young drivers can't afford to insure 16V or 20V turbos they might buy the NA models and then stick them in to trees, pushing up the accident rate for the NA models, making them more to insure.
That's my guess, anyway.