Showrooming is the primary demise of Best Buy/Future Shop.
Secondary. The primary cause is that most of what they sell is being rendered obsolete by smartphones and tablets. Stereo equipment, consumer cameras, movie and music in physical form, desktop (and increasingly, laptop) PCs, software... all the things they used to fill these stores with are vanishing as average consumers no longer buy them, or buy them through online channels (e.g. iTunes store, Netflix, etc).
That leaves TVs/home theatre, smartphones, tablets, video games, and a lot of empty space. Seems they've been making more of an effort to get into the small appliance business, but that market is already well-served by other big box retailers. They're running out of things to sell, and thus electronics stores just aren't a destination like they were in the 90s and early 00s. This is why Best Buy's fortunes did not improve after their main competitor in the US (Circuit City) went under.
Showrooming is definitely a factor, but more so in the US. Here amazon's offerings are far more limited. The bigger problem is people just don't spend as much on electronics as they used to.