Unless paid for, why will they spend extra resources to code for Nvidia’s custom implementation? Nvidia will have to pay I would imagine. Nvidia will need to provide something MUCH better than the RT that the 2000 series provides in order to stand out and have devs want to code for that without being paid extra to do so.
Because nvidia doesn't have a custom implementation. Nvidia supports the open standard in Vulcan and the MS standard in DX.
Your question is the wrong way round. Why would a dev support a non-standard customized API form AMD when they have a far smaller user base and would have spent the last few years using the Microsoft standard?