Because,Bright Spark,dinner is the main meal of the day, and you can't get more 'main' than Christmas dinner. It would be dinner if it were eaten at 8pm and is still dinner if it's eaten at 1pm. Whenever the dinner was eaten, the lesser meal that day would be supper or lunch depending on whether that lesser meal was in the evening or around 1pm..
I work early, so it's a cup of tea at 2am, a cuppa-soup at 5am, breakfast 9am (which tends to be more of a lunch, ie curry or lasagne), lunch 1pm -which I always skip - then tea at about 5 or 6pm. Tea is quicker to say than dinner!
Posh:
early 20th century: perhaps from slang posh, denoting a dandy. There is no evidence to support the folk etymology that posh is formed from the initials of port out starboard home (referring to the more comfortable accommodation, out of the heat of the sun, on ships between England and India)