I have often found in my experience that in fact Tescos especially when it comes to electricals and such like are not necessarily cheaper. If you take a look at the change in consumer habits, you'll find that these days, more and more people are becoming more consumer aware and are using the internet for shopping. Some high street stores offer excellent internet only deals that can't be obtained instore. Figures released over the xmas period have shown that although there might be more people out visiting shopping centres, they aren't necessarily buying.
If I'm going to buy say a new TV, then I might visit a shop to see the item physically, and then use various price comparison sites (that don't flood your email with tat ) and get the best deal online.
So I couldn't necessarily blame Tescos, I think the way we shop has changed.