That's a good question.
Perhaps there weren't any other better-qualified ops officers assigned to Voyager?Idk but maybe Starfleet felt that ops officers were not very relevant to the Maquis-hunting mission,like how having a counselor on board was unimportant too(Which I reckon is a bit silly since isn't the ops position important too?).It is a bit hard to believe or imagine that ALL the other ops officers were freshly out of the academy too,and that Harry was the best out of all of them.
I suppose we could rationalise it away as Harry was top his class in Operations Management or it was his specialty at the academy. Janeway seemed to have an eccentric taste in officers when picking her senior staff.