This is a great question and there is no one particular reason. As you can imagine, all managers are different and all situations are different as well. In light of this, let me offer some possible reasons.
First, the problem can be with the manager. He or she might have a personality that does not allow for delegating responsibilities. It is possible that the manager is a workaholic or obsessive compulsive when it comes to work. Or the manager can even have a personality that micromanages things in such a way that he or she has a say in everything. So, the reason for the lack of delegation can be with the manager.
Second, the reason can also be potentially found in the people of the company. Perhaps, the people that the manager has are truly incompetent and the project in view is so important that he or she needs to do it all under very close supervision. This is also a real possibility.
Third, another possibility is that the manager has a new team of people and these people do not yet have the skills necessary to work independently.
In the end, there are too many variables to make a conclusive point without further information.