Is there any reason why Milwaukee was chosen as a city to have nonstop flights to Los Angeles other than the fact that it was a hub for Midwest?

I don't understand why you would expect any airline that doesn't have a hub or at least focus city in Milwaukee to provide nonstop service to Los Angeles.

You're looking at it in the wrong direction (from the other companies' standpoint). Why would Milwaukee be a priority for nonstop service from Los Angeles for any cash-strapped airline?

And I think you're dreaming if you're listing NWA first. NWA provides no nonstop service from Los Angeles to anywhere in the US other than its hubs (and one mythical flifght to Las Vegas which is there only to provide connections from Japan). I don't think Airtran does either. United does, but to a high degree only in the West.

Meanwhile, be careful what you wish for. Southwest doesn't go into any city unless they can provide 8-10 nonstops (to a mix of distinations) from there. So likely if you see Southwest going into Milwaukee it's because they're ready to kill off Midwest. (And Southwest has been discontinuing long transcons like Los Angeles to Baltimore, so even if Southwest went into Milwaukee, Los Angeles would not be on its list of top priorities.)

It seems to me that Milwaukee's problem is MDW. If there wasn't an MDW, LCCs might be viewing Milwaukee as the alternative airport (to ORD) for Chicago, the way Southwest uses PVD or MHT in place of BOS. But with MDW available as an in-Chicago airport for LCCs, there's much less demand for a secondary airport as a substitute for Chicago than there is for secondary airports in cities an hour or two away from other major cities.