Because I had an iui with multiple eggs released. They were just making sure how many babies there were. I also got released to a regular ob since everything looked good. I should have another one done at 8 weeks and 12 weeks too. I think it depends on the office. My sister in law went to a regular ob but did Clomid. She got an ultrasound at 6, 8, 10, and 12 weeks.
Also, I am in the US. Tennessee to be exact. It may be different in other states and countries. I do believe that if I had conceived naturally, I would not have had one until 8 weeks just to ensure the baby implanted correctly.