I think that they conduct different stages of IEO because they cannot collect necessary sum of money on the one exchange.
It became very popular to have different stages of IEO on different exchanges. It helps to collect more money and more people will know about the project, because after IEO there is a listing of the tokens.
Now many cryptocurrency exchanges conducts IEO. The choice of exchanges is very big. But it is necessary for exchanges to have a big liquidity and a lot of traders.