Because ISPs are terrible at picking winning technologies that are worth paying for. If they provided all their users with a search engine 10 years ago, it wouldn't have been google, it would have been some lame competitor.
It could just be a very simple setup:
1. webapp notices you're on ISP A
2. webapp asks ISP A to bill you $3/month
3. you get an email from your ISP asking for authorization
4. you click the link and it's added to your billThere was a time that this would have worked, but I think there are too many people that aren't tied to a single physical location for their internet services. And we haven't all switched to something provided by a cell company.
It works just fine and no need to get the thousands of ISPs that exist involved, which is incredibly messy.
Going the ISP route is far cleaner, since they already have a billing relationship with the customer.