I'm thinking that quite possibly the carriers consider yield on a particular route proprietary information for marketing. Yield may change according to time of day and type of equipment (capacity vs costs/fare choices and percentages available of each fare on each flight) used. These often are closely held so that internal choices can be made to all the above to get that magic yield number at it's highest. Thats Yield Management and they have invested quite a tidy amount in that department.
Less sarcasm and more thought equal better understanding