They are both trying to monetize the content. I thought that was obvious.

That is obvious, but the person I was replying to said "if you don't want the traffic." It is not evident that they want to exclude 99% of web traffic, only the relatively small number who are unwilling to view ads.

In short, for most people, a paywall is a much stronger negative than ads, so it's not an obvious why somebody who wants to use ads should just use a paywall.