Probably a combination of durability and looks, until it gets dinged, or corroded.

I've stripped 2 of the 4 chrome plated plated frames I have. Both had corrosion under the paint that was virtually impossible to see when painted. I'm not sure if the chrome plating helped or hurt in that regard.

I do know that my powder coater much prefers to work with bare steel vs. chrome plated frames.

I think it may have something to do with protection from alien radio waves, but I've got a helmet for that.