It's a good question. I can only guess about some of the things that go into a meet-and-greet. Casting is definitely one as you mentioned, as well as training; the characters (especially those who talk to guests) need to understand the character in order to act like him or her. In Central Florida, temperatures are a factor. A person can only stay outside in some of these costumes for a very short time, so coming up with strategies for cooling, scheduling breaks, and even having two people in costume at once so they can be quickly switched out are all considerations. There's also a commitment of cast members to man the meet-and-greet and a commitment of park space which often has to be themed (characters don't just pop up spontaneously on the street as much anymore). Scheduling also has to be completed well in advance, so that the character's hours can appear in the Times guide for the park and on
MDE. And all this assumes that there is already a costume designed for the character, but designing, creating, and maintaining costumes are all considerations also. I'd be interested in knowing how much it costs just to bring the costume to fruition.
Characters also wax and wane in popularity through time. There will always be a demand for Mickey and the rest of the fab 5, but how much demand is there for a character like Wreck-It-Ralph right now? My DH would love to meet him, but I don't think he would experience the same lines as Anna and Else, Mickey, or the princesses. I'm not sure if it's still there, but there used to be a cute meet-and-greet location themed to Monsters, Inc. in DHS (I think it was near the backstage tour, so it's probably gone). The space was there, the characters were there, but the crowds weren't. The meet-and-greet was providing another attraction for the park, but to what end if few people bothered?
