Because making games compatible for both wastes the power of the current gen consoles.
For example if you did buy a ps4/x1 you'd want games that use the power it has, instead of feeling like you wasted your money because the games still play the same way they would on your old console
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This. We cant hold it back forever because some people don't want to make the jump. It's just not fair to the majority who want more out of Destiny