Most often it's because I haven't been asked.
The European Tour was put together by my literary agent's foreign rights person talking to all the European publishers of Coraline last year and earlier this year about when they were publishing and whether they wanted me to come and promote the book. Lots of them said yes, some said no. (The Norwegians and Finns and Swedes asked if I could come back in the autumn, and we'll see if it's possible -- I hope so.) Sometimes a publisher doesn't want the expense involved in bringing me in, which is understandable.
Sometimes it's because, although I've been asked, I've already comitted to something else. (I was invited to two Australian Book Festivals this year, but invited at a point where I'd already agreed to be in other places, and I asked if they can invite me again for 2004.)
And sometimes I'll say no (or, quite often, Lorraine-my-assistant or Merrilee-my-agent will say no for me, and not even check with me) because I can't go to every convention that asks. If I did, I'd never be home, and I'd get precious little writing done, and then nobody would have any reason to invite me anywhere.
If you've asked and I've said no (or someone said no on my behalf), ask again, in good time, for the following year.
It's March 2003 right now, and 2004 is already starting to be locked down.
I always wondered what happened to the suitcases that airlines lost. It seems they get auctioned -- to people who can't open them to see what's inside. There's a lovely account of it here: http://www.observer.co.uk/travel/story/0,6903,910287,00.html