Well your ISP knows more about you than your VPN necessarily does. Your ISP probably has your credit card on file, with your real name, and they have your precise street address too. The VPN may have none of that, except your IP address. If somebody were to purchase your history from your VPN, they would have to also purchase the IP->name/address/etc mapping from your ISP and JOIN the two. That seems marginally better than a one stop shop.

(Of course, some people give their VPN their credit card info, so the above rationale doesn't apply for them.)