There's a few reasons:
1.) Lower ownership costs. Since its a plastic bumper, there's no need to repaint if it needs to be repaired or replaced
2.) It's meant to give it a more rugged appearance (alla Xterra). Whether you agree is a matter of opinion.
model not only gets color keyed bumpers, but a color-keyed hard spare tire cover. The LX
and EX use a black rubber cover.
The CRV is a pretty impressive package for the price. I sold a lot of them when I was in Arizona and even more now that I am in Jersey where the 4WD comes in handy.