Feeling overwhelmed - you are feeling so overwhelmed and under stress, you don't have the capacity to look up and objectively access that the best course of action is to delegate.
You are rushed and have immediate deadlines -you are so behind on your deadlines, and you feel the pressure to get the job or project done immediately, therefore you don't feel as if you have the time or energy to delegate.
You don't trust that others can do it as well - you think that no one can or will do the job as well as you can. And since you feel that because you are the one that'll be evaluated on the job's performance and it's execution, you don't want to risk putting your own reputation at risk.
You've been burned in the past - been there, done that, got the t-shirt. You've tried to delegate in the past and it hasn't worked. All it's proven is that delegation costs spending a lot of energy with very little positive return.
You've never seen it properly modeled - you hear about delegation all the time, but you aren't sure what it looks like and therefore without any clear vision or model of delegation you are uncertain on how to proceed.
Perhaps some (or all) of these barriers mentioned above,are keeping you from delegating. Recognizing and being honest about them is the first step to addressing them head on - because as a leader it is essential that you delegate.