This is really interesting, and I think it might be similar to why we don't generally have snow tires / chains etc as a common item here in stores. Certainly the South rarely gets snow, with Wales, The North and Scotland being more likely to get snow days.
From the MET Office:
The UK gets on average 33 days of snow fall or sleet a year (1971 - 2000). Most of this is snow falling on higher ground where temperatures are lower.
Also from the MET office:
How much snow settles?
Much of this snow fall does not settle, and the figures for snow on the ground (snow lying) are much lower. On average across the UK there's only 16.5 days a year when snow is on the ground, compared to 27.7 days in Scotland. Again, most of the snow on the ground can be found in mountainous areas.
Without going too in depth into marketing... stores are very likely to sell what they know a consumer is likely to buy, and in numbers. If you'll note snow shovels and sledges turn up in UK stores in winter and certainly in the South they are plastic, and not likely to be used (or last if they are!). They will sell well when people are going to panic buy and therefore the quality can be low due to an unlikeliness to sell but not many people will panic buy snow shoes. The stores we have would prefer to cater to weather they can guarantee we will have... rain is more common all year round - you're more likely to find waders than snow shoes in our stores.
Ideally if you want something that companies like Blacks, Millets (acutally owned by Blacks), Sports Direct, Go Outdoors, Trespass etc might consider specialised, you're better off buying online as they might not keep the items in stock, as here I would say they would very likely come under seasonal stock.
As a note Blacks (and others) have 'Snow Boots' but they certainly look more fashionable than servicable, being more like very winter wellies... So lack of continuous snow and availability would be why people are less likely to use them.
MET Office UK Snow