Crossfit is popular because it's "easy". It fits the image that people have of what a gym does. Compared to normal free weight training, machines and crossfit wods are easier to do and look like more fun or entertainment.
Lets face it. 90% of people who go to a gym do it for ego reasons, not to train for a sport or to be a athlete. They are motivated by guilt and ego, which is why they tend to give it up once they realise it takes consistent effort.
Beware the popular. Beware the easy.
For example. Who are the most popular people on the planet?
What are the most popular types of food?
What are the most popular types of entertainment?
What are the most popular life goals?
In general, the fatuous, shallow, vain and brainless are favoured by the masses, along with easy food, easy living and the pursuit of comfort. The capitalists just love it. People live their lives consuming, while those that fall off the edges and become disillusioned become suicide bombers.
So, whatchawanna do today? Obsess about the new royal baby? Follow Kim Kardashian on Twitter? Eat a pizza while flagellating yourself during The Biggest Loser? Or go do some deadlifts that strike fear into your heart.