Well, I think there could be a number of factors involved. Hallowe’en was never such a big deal in many parts of Britain anyway, and in other parts of Britain we already had our own traditions and customs associated with the time of year, for example the traditions with apples and lanterns (usually made of turnips). Historically, the Protestant Reformation saw a lot of the Medieval Catholic festivities completely eradicated too. In the places in Britain where folklore customs survived, a lot of traditions persisted up until the modern era, but with modernity and greater mobility, many of these traditions have withered on the vine so to speak. In addition to those factors, the weather in Britain in late October isn’t that great and on November the 5th there’s Bonfire Night, which is much more of a communal festivity. The modern, commercialised Hallowe’en can also be a bit grotesque at times too, it might not be something that a lot of people want to be involved with.
Just for the record, we always celebrated Hallowe’en with apples, apple bobbing, a turnip lantern on the windowsill and a few old family ghost stories, but we didn’t go trick-or-treating. However, I was allowed to “trick-or-treat” my grandparents, but that was it! 😀