If the disk scheduler had known all the disk requests priori then it could have scheduled the order of servicing the requests. That is, for FCFS it would service in the same order as the requests came. For SSTF the disk header services the request which is closest to the current one(in other words where the header movement will take the shortest time to reach).
In this current scenario, the disk scheduler doesn't know what is the next request(as they come one by one after each request gets served) so it can't manipulate the movement of the header and therefore has to move as per the order of incoming requests.
This was my understanding. Correct me if I am wrong.