Some people have ATI cards, some people have Nvidia cards. Everyone with a gaming PC and Windows will have DirectX. Physics is one of the "next big things", and Microsoft know this. Whatever physics API Microsoft build into DirectX will be the one that developers use, simple as that. Developers want an open, well-documented and well-distributed API, not several proprietary ones.
It's then up to the hardware manufacturers how they add their support for that API.
And then we'll be back in the world of ATI vs Nvidia benchmarks for directX physics, arguments, fanboi-ism, mine's-bigger-than-yours/but-mine-cost-less-than-yours etc etc etc. Nothing much changes.