Doing this would be mean filling hour blocks partially in multiple colours and this would make the text unreadable on many devices.
The hour numbers are pretty much as small as I can make them, while remaining legible. On a standard (for most people) 400 pixel width display, each hour block is only about 16 pixels wide. At this resolution, changing 4 pixel columns of background colour for a 15 minute increment is pretty unnoticeable, but it does make the hour text much harder to read.