It's because Greco-Roman and Freestyle wrestling are widespread sports with millions of participants around the world. Just in the U.S. alone there are over 250,000 participants in high school wrestling each year. That's a huge talent pool to draw from.
In contrast, catch wrestling is almost non-existent as a mainstream sport. Freestyle wrestling is basically catch wrestling with the submissions taken out. Some years ago the mainstream of amateur wrestling went down the path of grappling without submissions. Some catch wrestlers went into professional (I.e. scripted for entertainment) wrestling. Others taught in relative obscurity. It's making a bit of a comeback now with the renewed interest in submission grappling due to MMA and BJJ, but there just aren't enough qualified instructors yet for the numbers to be very high.