Now that I think about it...
Platform exclusive deals are a weird gamble of publisher saying: "I bet your game is so good, it will pull players to our platform and make up for this investment in the future", and developer saying: "I bet that you are overestimating the value of my game, and I am not loosing on potential sales on other platforms." ...
In reality developers will be attracted to reliable platform offering quality service. A good cut also helps.