I guess it goes with the theme of light/good guys uniting for a common cause, destiny bringing certain people to together and all that good stuff. Also, nostalgia and because they probably didn't know who else for Aqua to make a D-Link with.
Not that Ven had much more of a connection with Donald and Goofy, mind. I think the common factor in all this is Mickey though. Aqua and Ven are both friends with Mickey, who Donald and Goofy are desperate to help.