Why do doctors have a hard time prescribing me xopenex (levalbuterol) hfa (an asthma inhaler)? Some does not even know what it is.

Not sure... Some insurance companies have this inhaler on formulary and others don't. If your doctor has not heard of this inhaler, he/she can look if up so it is not clear to me why they would not prescribe it. In some patients, it causes less of an increase in heart rate so it is sometimes better tolerated. Your doc could get more info from your pharmacist if needed as well.
Many choices. The choice of a particular bronchodilator to use for a patient is based upon: the physicians experience with the medication, the patients experience with the medication, other medical disorders that may effect the choice of a medication, the cost of the medication, andformulary coverage.