There is also the 'player' factor here. Many of the players are young enough they do not remember McD as an active player in the commercial aviation world. They also don't see the airlines they idolize or want to emulate using McD aircraft. Other than a few airlines using them in Europe, American(who probably suffers more bad press than any airline really deserves), Northwest and Delta. There simply isn't a significant example for fans to want to model.
How many McD aircraft are being operated in the Austra-Asian environment at this time? The game's sizable fan base in these areas would have a smaller experience base with McD aircraft.
In Europe i can think of only SAS, Alitalia, Spanair and Meridiana, and a handful of small regional carriers operate the MD-80. I believe Iberia has phased out theirs. Blue1 (SAS) operates the 717. KLM operates the MD-11.
In the US American, Delta and Allegiant all operate MD-80s, while Delta also has the DC-9 and MD-90. FedEx and UPS both have tons of MD-11s and MD-10s.
In Asia? The Asian boom only started after McD was in the mud, so almost none have been sold. There was the infamous trunkliner program to build MD-90s in China, but Delta has bought most of them. A handful of MD-80s operate with the six Chinese majors, and a scattering of LCCs and small airlines all around Asia, with most being phased out. Biman Bangladesh still has a few DC-10s, but they will be gone when the next few 777s arrive. JAL has 16 MD-90s.
In South America there are still a few left in Argentina, but thats about it. At least no major airlines operate them, apart from Avianca and Austral/Aerolinas Argentinas, but they are both phasing them out.
In Africa. Many all around with blacklists and other nasty surprises hanging on them. Also quite a few in South Africa, which should be safe enough.
Middle East: Saudi Arabian has/had 30 MD-90s. Being phased out. A scattering of DC-10F and MD-11s (no passenger bird though) all around.