It may have been for entirely aesthetic reasons or it may well have been to avoid some of the topweight issues which were a problem with the liners which had the two deck arrangements. The heavy woods, wrought ironwork, and stone columns may have been pretty, but they did some not so nice things to the ship's stability.
That is quite attractive, Luke--I'll have to go back and look, but I think the Amerika's dining saloon was set up something like that, but hers was much too overdone; I like the simpler, more tasteful styling that Olympic & Titanic used.
For all I know, the White Star Line was quite eager to make all the possible effort to make the most space for cabins. Thus they tried to reduce on "public areas" all they could. Maybe this was one of the reasons for making it like they made it.
Also to adding to all this 2 story dining room pros and cons. I know a con. The first class dining room was on D Deck right? Well, if the bottom story was on D Deck, wouldn't the corridors and first class staterooms above on C Deck be affected by the fitting, and plus , more racket to planning the ship.
Good question! But they was no need for a 2 deck high dinning room, the room was so big it could already accommodate all the first class passengers in one sitting. The 2 deck high dining rooms on the Cunard ships may look impressive but was seen as old fashioned by the time Olympic was built. The large open spaces would of been hard to heat and It was far more sensible the use the space for more cabins or luxury facilities. D deck was also taller than other decks so it would of felt more bigger than on other decks.