What I know about MAW is from reading a lot of articles, going on their website and talking to some people associated with MAW. So, I am not speaking for MAW or any other wish granting organization.
From what I know about MAW, it is a national organization, but each chapter of it has some autonomy in how they do things.
They don't have money to grant wishes to everyone who comes to them, no matter what their condition or need. Being referred to them is definately not a guarantee that the wish will be granted, so anyone who does apply needs to realize that it is just a wish, not a sure thing until there is a contact that says the wish has been granted. I do know that their highest priority is granting wishes to kids with terminal or immediately life-threatening conditions (since they may not have a second chance for a wish). After that, they go on to other "life-threatening conditions" (per their web site, as Mike noted) if the individual chapter that serves the area has money/resources left. Depending on their resources and their board or directors, what one chapter may consider a life threatening condition that can have a wish granted may not be considered for a wish in another area.
As Mike suggested, you may want to check for other wish granting organizations in your area. MAW is the best known, but they are not the only ones. Some have differnt requirements and will grant wishes to kids with chronic conditions that are not life-threatening.
I also agree that if there is a question about why a wish is not granted, it should be directed (nicely) to the people who made the decision. The MAW people that I am aware of are very dedicated volunteers who are doing the best that they can. Sometimes they have to make hard decisions, based on the requests they receive and the resources they have. But they are in the business of making wishes come true (as much as possible), not giving out false hope.