A student reader wanted to present their thesis research results at a conference, but the student's advisor said no -- he would present the results, and the student couldn't even go to the conference. The student wonders why he/she couldn't give the talk or attend the conference.
The easiest way to find out would be to ask the advisor, but in the absence of this information, we can muse about some possibilities. Please add to the list if you know of other possibilities. I think it would be most useful if we confine ourselves to examples we know have occurred, but if you want to throw in some paranoid speculation, go right ahead (though it would be helpful if you noted that you are just speculating.)
1 - The advisor is on the tenure-track or otherwise needs the exposure (see point #4 in GMP's recent post). The student may need the visibility as well, but if the advisor doesn't get tenure, that isn't in the student's interest either. If this is the case, the advisor should just explain to the student: "This is really great work and I think it would be best for my tenure case and for the research group as a whole if I present it at the X Conference." Ideally, the student will get due credit for their work, will have other opportunities to 'own' the work (at conferences and in publications), and should feel some satisfaction that the advisor thinks the work is good and important.
2 - No matter what the career stage of the advisor, there may not be enough money to pay for the travel of both student and advisor. Some conferences -- particularly if they involve international travel and high registration fees -- are very expensive. If there is only enough money for some, but not all, people in a research group to go a meeting, the advisor will make decisions, some of which seem (or are) selfish. If it is important for the work to be presented (e.g., to show progress on a grant-funded project), and the advisor definitely has to go to the meeting (to chair a session, serve on a panel, attend a meeting-within-a-meeting, schmooze with funding program officers), the advisor will go and the advisee might not. Again, the advisor should just explain: "This is really great work, but I only have enough travel money for one of us to go to the X Conference, and I need to go for [these other professional reasons]". Note: As a a grad student, I paid my own way to some conferences in the US when I could afford to because otherwise I wouldn't have gone to the most important conferences in my field, so a separate question is whether the advisor would/can forbid a student to present their own work if they pay their own way.
3- If the advisor thinks the student won't do a good job with the talk, s/he may decide to give the talk to make sure it is presented well. This decision may be based on experience or speculation. If this is the reason for not wanting the student to give the talk, the advisor should be clear about the reason and proactively help the student improve for the future. (Yes, I know that advisors can and do give poor talks as well, but we are discussing here why an advisor might choose to present a student's talk instead of the student).
If the advisor is giving the talk on research primarily being conducted by a student, it may not be possible for the student to attend the conference if not giving a presentation. Depending on the source of the travel funds, their use might be contingent on active conference participation.
Mostly, I think the advisor should explain the situation and reasoning to the advisee, whatever the reason. And if that doesn't happen, it would be good if the advisee could ask for an explanation in a non-confrontational way. I know from the e-mails that I get from students that some are very reluctant to ask their advisors these types of questions, as if asking implies criticism, and perhaps fearing negative consequences of some sort.
These students may well have experience that shows this to be the case, and if so, perhaps the information could be obtained in an indirect way -- by asking more senior students, postdocs, or friendly faculty who are willing to explain some of the more mysterious aspects of professorial behavior and decision-making. In other cases, advisors might be happy to explain their decisions, and it just didn't occur to them that there were questions and confusion.