The reason? Styrofoam has polluting chemicals in it and has a tendency to stick around landfills and other things as they're not biodegradable. That is bad. The ban is probably going to pass as nine out of eleven Supes voted for it. It also has the approval of most restaurants.
Now before you roll your eyes and say "what are we doing now?" keep in mind we're actually behind the curve on this one as over 100 cities have already instituted such a ban. Berkeley and Oakland have already passed the measure. Come on, people, we're slacking!
The city will come up with a list of allowable materials-- look for boxes and/or waxed boxes-- and send them off to various restaurants so they can plan ahead. Between this and the plastic bag ban, ordering things is about to be a whole new experience.