I suspect that what I see is a skewed sample, that perhaps people who have time to post things 2nd class are more likely to already have stamps. We still get a lot of things posted 1st - the only ones for which 2nd is the most common seem to be ebay sellers sending out parcels.

People who come in to buy larger quantities of Christmas stamps are more likely to buy 2nd, compared with my experience of the rest of the year.