My take...
Years ago, when launched, Maplin were a components shop, supplying a hard core of hobbyists. I was one. I daresay there’s less demand for that now. For example, people just buy a raspberry Pi, rather than buying all the bits and making one for themselves. Plus, years ago, higher end component suppliers like RS were trade-only, nowadays RS and others are much more accessible. And why buy and etch your own PCBs, when there are internet suppliers that will manufacture one-offs quite cheaply?
In last couple of decades Maplin seem to have reacted by trying to find a new customer base, selling more and more gadgetty and PC stuff, often stuff that was already obsoleted, or just useless tat, competing with everybody from gimicky toy stores to PC World. I am not surprised that tactic failed to save the business. Sad, though.
Or of course, they could just blame Global Warming, Brexit, Diesel exhaust fumes, or whatever