It seems to me that if they allowed a LFG channel to be available worldwide it would just switch the problem from LFG people in Trade to all the Trade people switching to LFG. After all, why use a city only chat when you can spam your merchandise to the whole world?

I'm not sure whether or not I'd really care if there was a worldwide channel being used in such a manner, but I think Blizzard does, otherwise trade wouldn't be city restricted to start with. Not giving gold spammers the ability to spam a worldwide channel 24/7 probably also factored into the decision I imagine.