Because IP addresses are not a good way to track individuals.  NATting means that any individual IP address could be masking dozens, hundreds, or even thousands of unique people.  If you implemented a metered ban of 10 articles and 10,000 people have the same public-facing IP address, you would blow right through the limit in a few seconds or minutes and present the paywall incorrectly to X numbers of users for the remainder of the metered period.  That's fairly user-hostile behavior and not recommended as a brand-builder.

On the other hand, you are willing to live wih a certain amount of "theft" from the more technically savvy members of your audience because the vast majority of users don't have a clue how to get around the meter.  Better to "lose" a few subscribers to superior technical knowledge than lose an entire multi-national company's worth due to poor decisions.