Because most people thinks that CP means how good/strong a mon is and Rhydon is one of the higher CP mons that is easier to get anywhere. I really smile when I see a gym with 3 or 4 rhydons, one single Vaporeon and you can beat them all easily, fast, and without wasting resources.
The only good thing about Rhydon for me is that is very easy to prestige against it.