Baby Gender scan? Confused

My next appointment is January 2nd. Ill be 15w6d so basically 16 weeks. Ill be getting an ultrasound done. They told me that they will send me when im 18-20 weeks to the hospital i will deliver at to find out the gender through ultrasound. Well, how come? Isnt there a possibility they will see it on January 2nd? I don’t understand if they have the availability to see it then, why would they send me somewhere else?