Your question is really valid, I read an article for someone requesting the addition of Revenue Management as a new project knowledge area.
Based on my opinion and my reading, i think that revenue management is kept intentionally outside project management to be addressed under portfolio management (which include operation as well), revenue is something more related to product life cycle, for example a construction company will deliver the building, but selling the units will be handled outside the project scope (under the realestate organization/portfolio), i think adding revenue management to project scope will add complexity (and value). I think we may see this in the future editions of pmbok