Because the vast majority of news consumers online are ad-supported non-paying customers.
I'm not going to pay $20/mo each to WaPo, NYTimes and WSJ. But I would pay $20/mo for "entry" level subscriptions to all of them, which I'm pretty sure is more than the Ad revenue that I would otherwise bring in.
$10/mo may not be the best price point, but IMHO having cheaper pay-as-go options would drive in a lot of unrealized revenue.