The overall objective of a company is to make a profit, there are several paths to achieve this objective, one of them is making the process for your making products cheaper so that when you sell them at the same price you make more money. Developing nations, have weaker employment laws, therefore minimum wage is less, and companies see this as an opportunity to make a profit, also in these countries there is usually a lot of unemployment so people will have to put up with these conditions. There are also other ways companies save money by outsourcing, for example, avoiding taxes in tax-free zones, acquiring cheaper resources from countries that do not have enough ecological regulations in place or simply by not spending as much in improving workers spaces and conditions.
I suppose it would be the lower costs- a developing nation doesn't have wage laws & therefor you could pay children rocks in exchange for work- though it's never at that extreme, there's not much stopping a company to do so.
