Why? Who knows. I don't think the reason matters...it isn't as though your cruise is being delayed or cancelled. Since your signature indicates you have over twenty cruises under your belt dating all the way back to the "Big Red Boat", surely you know what is involved on a cruise turnaround day and since most mainstream line ships are now sailing at full capacity, opening boarding at 11:30am is very reasonable.
While I've never understood the desire to be one of the first on the ship, if that is important to you then just show up really early and be prepared to wait.