More importantly why do all digital SLR cameras use a 4x6 aspect ratio rather than a 4x5?

It would certainly make it easier to buy picture frames if cameras used the 4x5 aspect ratio. On the other hand, some cameras provide the wider 9x16 aspect ratio, which is nice for the current crop of computer monitors.

It all goes back to 35mm movie film. Movie format was 18 x 24, so 36 by 24 was simple double frame. The 24mm width was pretty much the maximum that would fit between sprocket holes. There was another reason for the 36mm width - it was 8 sprocket holes and that had become a de-facto standard for slide projection after world war II.