The steel gas tank used to be required to pass a puncture test that the DOT requires on all street bikes. BMW challenged this test initially with the first big plastic tank they did on the R100GS/PD in 1990?, and like you mentioned, uses plastic on nearly everything they make now, except for the Roadster and Cruiser.
Steel tanks are cheap. With all the extra gear they have to add on dual sport bikes, they've got to save money somewhere. So it is steel tanks and inferior suspension components to keep the price down.
DOT requirements are probably the main reason, though the other reason is appearance. Steel holds paint MUCH better and you're not supposed to fall down riding on the street so dents shouldn't be an issue.
the reason why honda uses steel tanks is because of emitions. The plastic tanks lets vapors out though the plastic, (the reason why your stickers buble and turn yellow). I am not sure why everyone else uses them but my guess would be the same reason. Atleast this is what my friend that works for honda told me long time ago.