1) Early arrival or early check-in is NOT a request option.
2) It all depends upon Housekeeping
. . . is there enough staff to clean rooms
. . . is there a room available
. . . is it the room that was pre-assigned to you (if it was pre-assigned)
3) The Room Assigner ("Rooms" person) cannot know this in advance.
4) The bulk of housekeepers start cleaning rooms at 8:30am.
5) If there is a room available, it was probably vacant the night before.
NOTE: I know your question, "Why start cleaning so late in the AM?". Simple. Most people don't get out of the rooms for breakfast or park buses until about that time. Sure, there are few housekeepers who start cleaning earlier, but very few.