One reason might be the fact when you're a provisional driver you drive with an experienced driver next to you, ie more likely to drive safer. When you've passed your test, you're a 17 yr old let loose on the roads by yourself so you're a higher risk.
You know what, you'd be better off having insurance as a provisional, gain a yrs no claims discount, pass you test a yr later and take your pass plus, you should gain good discounts when you come to insure as a full licence holder then.