I'm going to quickly address SUNT's point that it's not done this way in other events and then be on my way. Here is a possible reason (although who knows if I'm right on this) as to why it would be done in the all-around gymnastics competition. The all-around competition is different from other events. Theoretically, you could put together a team of 4 girls who are strong at each event, but not great at each event. In the meantime, you have a girl who is great at the vault (i.e. McKayla Maroney) but not that strong at the other events. In a world where 4 girls from one country are allowed to compete in the all-around, you may not see McKayla Maroney make the team. In essence, by allowing more than 2 girls from each country, you could be watering down the talent in the other individual competitions and actually undermining the "best vs. best" idea of the Olympic games. This probably happens to an extent already.