I guess I'm not really a fan of chess.com's visual format, but I'm also very much not a visual person, so I don't really care. I like their content (specifically tactics trainer and chess mentor) enough to pay for a premium membership, which has made it (by default) my site of choice for correspondence play, and so I've never tried anywhere else.
If you're specifically wondering why I chose chess.com for this tournament, it's quite simply that chess.com is the only place I've ever played correspondence chess, and I knew that chess.com has the option to create a custom invite-only tournament that would allow me to run this. Could I perhaps have run it elsewhere as well? Probably, but I don't really know, and that's why I didn't