They left Microsoft on bad terms and made a new partnership with Sony Online Entertainment which is why they are supporting their console. However, they do not want to leave the Xbox community out in the cold which is probably the reason the game isn't a PS exclusive and why they made it for all 4 consoles and not just the new gen. They are gaming developers first and put money second.
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Correct me if I am wrong but I am pretty sure this has more to do with Activision and their partnership with Sony, not Bungie.
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Money second. LOL.