The vast majority of digital signage in the world is Windows based, and as some have already pointed out, it's the driver situation that pushes companies to WIndows. A good chunk of signage software out there relies on the ActiveX subsystem to run, as a result, it would be a significant undertaking to pick up and move to another platform.
Digital signage is another area where hidden complexity makes laypeople think that they could implement their own solutions with nary a bead of sweat on the brow. In practice this is far from the case and high quality signage is expensive and complex. All that said, nothing like some lazy implementation engineers to ruin an otherwise good product.