InterWorld was never supposed to be a book. It was supposed to be a fun Television adventure. When Gaiman and Reaves shopped the idea to all the big name TV execs and got turned down, they decided to write it out as a book in the simplest terms possible - you know, so even suits could understand it...

Then they shopped it again.

The suits still couldn't get a grasp on the concept of a multiverse, and multiple versions of the same boy being able to traverse all of these worlds.

I guess the idea that Science versus Magic was too steep for them to grasp. Or that all these 'Joeys' would run around...