I think people rely on the 'experts' at B&Q and get bad advice.
It was highly publised last year that B&Q were employing people of older generations who had worked most of their life in specific trade and were indeed experts in their fields. This sticks in alot of people's minds and they go there beliveing that they are speaking to the expert, when infact most of the time, the person there speaking to either worked in a different field altogether and is in the wrong department or never worked in a trade at all.
I went to get an old imperial to metric connector a while back when replacing a very very old bathroom suite, the guy i spoke to made out to be a plumber and sold me the wrong part, i went back the week after and spoke to him about another plumbing part, asking for his advice, his reply - sorry mate i work in the electrician / electrics department. Obviously forgetting that he had professed to be a plumber the week before.
People are going and being told that the tiles are fine and that B&Q addy etc is as good as any other, they see B&Q as a massive DIY retailer and believe what there told. They buy believing that there as good as any other but B&Q can sell them cheaper, maybe becuase its a massive retailer, maybe they dont think why and just see the price savings.
No disrespect to anyone who works at B&Q by the way, just my opinion as to why people buy from there. People trust big brand stores.