The reason you don't use WhatsApp is that the people you communicate with don't use WhatsApp. The reason they don't use WhatsApp is that the people they communicate with (this includes you) don't use WhatsApp.

This is called "network effect", where the value of the product for you is amplified as more and more other people use the same product. You can get a value out of a microwave oven, even if no other people ever use it; you can't get a value out of WhatsApp if other people don't use it, since there's not much value in WhatsApp-ing yourself.

The question, then, becomes: Why the US doesn't get value out of WhatsApp?, and this has to do with history.

As people started to use messaging (only SMS existed at that point) instead of calling, the US carriers responded by increasing the basic subscription price while including unlimited messaging (SMS). The relatively rich US population could bear this added cost. This, however, was not the situation in pretty much the rest of the world, where the SMS was either an expensive "added value service" (not good for the customer), or "unlimited within provider" (again, not good for the customer who has to guess whether their SMS is going to be free or not).

As of 2010, when WhatsApp started gaining popularity, its value in the US was questionable, since no-additional-price unlimited SMS was a viable alternative (as it still is). The rest of the world have seen a tremendous value in unlimited and free alternative to the expensive SMS. (WhatsApp pricing was $0 for unlimited messaging in the first year, and $1 per year after that.) With that value, the rest of the world switched swiftly. Not all of them went to WhatsApp, as Viber seems to be the most popular in Eastern Europe and LINE in Japan.