They ply their trades elsewhere largely due to money. Many of them have admitted it, in fact there was a recent article/tv show about it. Can't remember which.
The quota system is necessary in all walks of south african life due to the poor conditions the indigenous african people were put in and still survive in. They need a helping hand, it's wrong to state otherwise. Plus, every time south africa lose, the quota system and by association non white players are blamed and yet, when they win...what then? Did the quota system just stop existing? It's also important to remember SA have the 3rd or 4th highest win/loss ratio in test cricket history, one of the best test sides in recent years and a very strong ODI side...let down by a choking mentality.