It’s a licensing requirement – they’re able to open in relatively affluent areas without being considered “an off licence” because they agree to it being a condition of their licence that they will only sell a minimum of 6 bottles.
Supposedly it discourages the ‘casual’ drinker believed to cause problems.
Of course, it encourages the middle class drinker (like me) that bit closer to liver failure.
I ended up sacking Majestic off for the Wine Society – I found M’s selection below about £15 p/b to be a bit ropey, but the WS stuff is on the whole superb.