I suppose some think it is inevitable because MSN are the only ones (of the big three) with their own browser. "Default search settings" tend to remain "default" settings by ... well ... default! :)
Most people are unaware that they can change their default settings or that they can download other browsers. Fewer still feel the need to do so if they are getting reasonable results. Ergo, if Internet Explorer is your great aunties default browser and MSN is set as the default "search" resource ... chances are that's the one she will use.
Unless Yahoo and Google get into the browser market, MSN has a huge leg up in the grab for eyeballs from the get go.