Insurance... They aren't considered necessary unless there is a risk (prior miscarriage, something seems wrong like unequal measurements etc). If you are progressing fine and don't have any prior history, your doctor probably doesn't see the need.
If you are really concerned, then call your doctor and ask if they will do one earlier. But if they aren't recommending it, you are probably fine. How far along are you? How have your appointments gone so far?
My first pregnancy I was three months when I went to the doctor and I didn't have so many ultrasounds but with this pregnancy (2nd) I'm getting one every other appt for being so early on in my pregnancy.
Re: Ultrasound