Tax and marketing sound like very plausible explanations but something of a shame that the brewers celebration of the season is coming to an end so soon. Having said that I've found a much wider variety and higher quality of Winter ales this year (2010/11) than ever before. There seems to have been a real effort made to mark the season which probably comes back to marketing but I'm certainly not complaining.
In the meantime, whenever I find a decent Winter Ale over the next month or two I'll be filling my boots and making a mental note to gorge well as soon as they appear next winter.