I already answered to you why. They had their chance when they played in the Argentinian League, and they weren't something spectacular to deserve a call-up to the national team. Gaitán played in Boca and Marioni in Independiente and their performances weren't enough to deserve a spot in the Argentinian team: Gaitán was released by Boca after a while, and Marioni was sold to a relegation battler team in Spain. When they played in Europe (Gaitán in Villareal, Marioni in Tenerife) they didn't exactly set the Spanish League on fire.
Now they are playing in a League that is of lesser quality than Argentina, and can't be even compared to the Spanish, so they won't be called up. The truth is that most Argentinians that move to Mexico aren't stars here, they are just good role players and nothing else, with the only possible exception of Cesar Delgado, but he isn't a fixture in the Argentina National Team either.
Still, the better Argentinian and Brazilian players are scattered in European teams. Both countries are exporters, they constantly sell players to the top European leagues, and always produce new talent to replace those who were sold. If you had to make a list out of Argentinian fowards, Marioni would hardly make it into the Top 20 (only based on current form), and that's for four or five spots for the World Cup squad. Gaitán may have a sweet touch and be called "El Divino" in Mexico, but he is nowhere near Riquelme, Aimar or Messi. He could fight for a spot with D'Alessandro or Veron, but only based on the poor form of those two, if they gain back their form, Gaitan has nothing to do near the National Team.
FIFA World Club Championships you say? Then why the is Costa Rica's Deportivo Saprissa getting their asses beaten by Liverpool if the Mexican teams are better? Brazilian and Argentinian teams have beaten European clubs before, so I credit them.
By the way, the South American Cup stinks, so far it's a tournament made only for money without any prestige (many teams qualify based on how good their TV ratings are, not how good on the pitch they were last season). The only real prestigious compe tion is the Libertadores.