We had to decide what to eat and what to avoid because it was poisonous, what to chase and what to run from. If we couldn't make fine distinctions in the natural world, we'd be done for. One speculation I make in the book is that our current consumer culture may be based upon the naturalist intelligence. You considered adding a spiritual intelligence, but ultimately decided against it. The more I investigated spirituality as it's used in our society, the more I became convinced that even if it's terribly important, it's not an intelligence.
Spirituality is just a mess intellectually.
Many people would say that what's important about spirituality is the feeling that comes with it. The problem is that we don't know how to measure people's feelings because they're not quantifiable. In mathematical intelligence, for example, we're interested in how well people can compute. How they're feeling at the time is irrelevant. When you start making a subjective feeling part of the definition, it gets very slippery.