“Private browsing” only prevents data from being retained on your computer. Those data (where you browse, what you click on, etc.) are still available to your ISP, as they are still transmitted in the clear.
Even HTTPS transmits some information in the clear. For example, if you log onto your bank securely (via HTTPS), the ISP does not know your username, password, balance, etc. But it still knows you logged onto that bank. Which is the information they can sell to marketers. And spammers.