The reason for this massive popularity comes down to the nature of the job. Truck drivers can't be outsourced overseas, and the profession can't be automated, though Mercedes-Benz is trying. Furthermore, there's a demand for drivers nationwide.
The job's popularity is especially intriguing when you consider that there's still a huge demand for new drivers. Retiring truckers just aren't being replaced, and the industry needs an estimated 239,000 people behind the wheel of big rigs by 2022.
Planet Money admits that that the data doesn't paint a perfect picture, though. The data is skewed because it includes drivers whether they deliver goods locally or haul stuff state-to-state. Conversely, the Census Bureau sometimes gets granular with its categories, like breaking teachers into primary or secondary school categories. The site also excluded data for some categories of professions because the groupings "are broad and vague to the point of meaninglessness."
Check out Planet Money to see the site's interactive infographic and watch the changing popularity of various professions. There are some interesting trends on display like the rise and fall of secretaries as a major job.