Because the iBook is tageted at the consumer demographic and not the Professional level. Firewire 800 is cool and all, but how many devices really see that much benefit from it? And fo the devices that do, how many are targetted at consumers and not professionals?
Don't worry though, if I remember correctly, when Firewire first came out it was only availabe on the PowerMacs at first and then filtered down the line. Eventually it will be in all of them. Patience.