They are the biggest because they provide the overall best and essentially fastest level of DDoS, geoIP block, and packet-inspection malware protection of any provider on commercial hardware short of utilising spooky predictive DARPA machine learning algorithms that ride the razors edge of sapience on government funded terawatt supercomputer clusters. They are expensive and you get what you pay for.
That’s exactly why many of us dislike cloudfare. They’ve maneuvered themselves into a “too big to fail” position. Seems to be the goal of big corps these days.