Consider this: When everyone (again) seems to prefer Boeings, what are the odds for you to get them from now on when you want to get another aircraft? Because of the fleet commonality, if oyu first picked up a boeing, you should stick with it. But, again, since everyone else has done the same, it's very likely you won't get another boeing when you want. Therefore, consider yourselves lucky when you didn't get the boeing, and pivk McDonnell-Douglas (I presume also Airbuses are out), and you'll have a nice jet with nice fuel economy, and you are most likely to get new DCs whenever you need, thus it's easier for you to expand and get more planes than those who picked the Boeings.
Also, if you get a good start, you can quickly afford a brand new Boeing, whilst those, who did get Boeing in the start, either have very few Boeings (planes) or have many different planes, thus high fleet commonality costs (maintenance, staff, training...).
Well, this is a bit simplified, but that's how t goes at least for some of the players