Offset printing does not require anilox rollers to determine ink volumes. The volume of ink can be adjusted across the width of the web depending on where it is needed by opening and closing the ink keys. This means that only one print unit is required when producing both solid blocks and fine detail with a particular color in the image.
Another reason for offset needing fewer colors, is the possibility to print in an extended gamut. By doing so, you can cover 94% of all pantone colors, which dramatically reduces the amount of different colors that need to be stocked.