Perhaps because license plates in the US are fairly small (at least compared to the UK for instance) and not all states require front plates?

That's exactly what they do now. Identifying a color on an LED is easier than reading the license plate of a car across the street or distinguishing one green sedan from another.

… unless you're colorblind.

They do that, but tag numbers are hard to read at night and from a distance. I'm in Seattle, the color bars are nice (though they should auto-assign them).