My perspective
I've walked into a Maplins, seen something I wanted to buy, seen the price of the item and walked out again without buying it.
Simply put they were too expensive. Not just high-street-markup expensive, but taking-the-piss expensive. Sometimes 5-10x markup, especially on cables and AV plugs.
If their clientele were idiots then maybe they'd have gotten away with it, but I suspect most of them were technically proficient and quite capable of looking up stuff on the internet and ordering from there instead. Even their own website was expensive possibly because they were scared of cannibalizing store sales. Dumb idea.