Why is this? Who the hell knows! It is stupid and, quite frankly, a discriminatory policy by NY state dept of insurance. There are other bizarre motorcycle insurance issues. The other big one is this: Unless you specifically request and pay for a rider on your policy, your spouse is likely NOT insured while a passenger on your bike. You could pick up a complete stranger, a neighbor, a grandson, some idiot off the street an they would be covered as a passenger, but not your spouse. You spouse is not able to sue to recover damages. You need to check with your agent.
WHY? No one knows... I suspect it was (is) some sort of backdoor pro-insurance anti-biker policy stuck in legislation 30 years ago as a give away from Albany to the insurance lobby.