Ok, so I guess I'm kind of venting here.

I'm feeling a bit agitated about a current situation and I figured that you all usually have some good insight...or at least commiserations. This is kind of long; but, I need to get this off of my chest before going in to work tomorrow.
I don't want to get into specifics; but, I'm feeling a bit jilted about a supervisor selecting a co-worker to be the lead on a particular project/initiative. I have no issue with the co-worker herself, I think she's a fine and likeable employee. I also don't hate my supervisor or anything, I just don't understand how management comes to their decisions sometimes.
The "issue" is that I have an expertise in a particular subject-area related to my job; a specialty if you will. My experience in this particular field extends several years back, to a time before I even started working for my present employer. Within my office, I am recognized as one of the "go to" people for this type of work, in fact, I am on the team for this subject area.
I am on a first-name basis with the faculty in this area and they frequently recommend the students directly to me for assistance. The faculty outright told me that they prefer dealing with me as they feel that I have a greater grasp of the topic. The students themselves also frequently request me by name and I recently found out that the students are recommending me to each other.
Because of my background in this area, I really took to this topic and have developed a deep understanding of the requirements, documentation, and procedures. Though they are "ok-ish" and do an earnest job, the other members of the team do not have the same depth of knowledge.
Not too long ago, my supervisor and I chatted about some of the issues we were having regarding this population of students. Towards the end of the conversation, she commended me saying that I was very good at what I do, acknowledged that I seem to have a significant amount of knowledge and experience with this area, noted that the students really seem to take to me and identified that I clearly have a passion for this particular area.
What she said next surprised me a little bit. She mentioned that she was a little concerned about having one person in the office who knows so much more than the others on a particular topic. My boss specifically cited a concern that, if I were to leave the department, I would, in effect, leave a "void". I acknowledged her concern and suggested that perhaps we should have a training session so that all of this project's team members could be on the same page.
We never did that. Instead, she appointed a co-worker to be the lead on the project (instead of me). That co-worker, as cool as she is, isn't fully up-to-speed. She and the one other employee on the project will ask *me* for help when they get stuck on a part they don't understand. It's a bit frustrating to know that I am more than qualified and yet my lead is asking me for help. At any rate, even though I was a bit confused with my supervisor's decision, I decided to be a team-player and have been taking direction from my co-worker without complaint.
A few missteps have happened with the project due to the other two co-workers inexperience or lack of knowledge on the topic. I feel bad because I want to stay humble; but, I feel frustrated. After all, my supervisor outright indicated that she felt I was very qualified, yet she chose someone else because she apparently doesn't want me to have too much power (knowledge=power). I get the concept of "spreading the knowledge in order to protect the office"; yet, logically, wouldn't an employer *want* to pick the most qualified person for a job?

I could have been the lead and would have gladly trained the others to make things more equal; but I guess that wasn't an option either.
Why do managers do this?
.....Kelly
PS - Thanks for letting me vent.