Simply because it's not one of the Sales Manager's Functions. It's an outside expert's area of competence, not a sales manager's. The intelligence, insights, suggestions and action plans that are included in the sales force evaluation should be put to use for coaching, motivation, compensation, selection, accountability, systems and processes, metrics, training and development, etc.. But the actual evaluating is not done by internal company management.
I also didn't include sales training or sales management training in those 10 sales management functions for generally the same reasons. Those are competencies best left to outside experts. Sales Managers can learn to coach - and it takes a long time to develop the ability to do it effectively - but training is a whole different animal. Training is not standing in front of your salespeople and teaching. Anyone with presentation skills can do that! Sales or Sales Management Training must include these 25 competencies plus these additional 10:
1. wide and deep subject matter expertise
2. ability to role-play the salesperson's part on any scenario at any time with anybody
3. ability to fully explain what is being role played, as it is being role played
4. the ability to create change
5. the ability to change the mindset, expectations and buy-in of an entire audience
6. the best, most effective tactics for any given scenario
7. tactics that support the company's strategies
8. the ability to make training participative
9. best practices from outside your company or industry
10. ability to overcome resistance
So in summary, just because it needs to be done doesn't make it an important sales management function. And just because it isn't an important sales management function doesn't mean you don't have to include it in your ongoing efforts to develop your sales force.