It was the only CTF map when the game came out. Everyone knows it, everyone remembers it, so casual players and returning players will often choose it. I played nothing but when the game first came out, but jumped ship when new CTF maps came out.
If you want a quality CTF map, play Turbine. Very difficult to hold the middle ground, so games swing back and fourth and get very tense. People are more likely to work as a team.