Why the South Pacific book excludes information on Nauru and Kiribati?????

Any reason for this?

Because, chile2004, Lonely Planet is going down the tubes. They used to have an entire book on the Solomon Islands and it was great. Now, they have 30 useless pages in the PNG book with no quality maps and info like "You can go to Isabela!" and "If you buy a map, you can do this!" Excellent, Lonely Planet! I ill happily give you money to tell me that I can do things without telling me how to actually do them.

It excludes all of the Micronesia region (where Nauru and Kiribati are) simply because strictly speaking it is not part of the South Pacific.

Micronesia now has the dubious honour of being the only region of the World not covered by a LP guide. Make what you want of that.
And get a copy of the old LP Micronesia for detailed (though dated) info on those 2 countries.