Well, I am not the one who should talk about the goals since I am not a ReactOS developer. But I can tell that NT 8.2 do not exist. You probably meant NT 6.2, on which WDDM took over the business.
I assume it's because of the backward compatibility. But it also would be because of the fact that to implement 5.2 APIs, they need XDDM. Skipping right from 5.2 to 6.2 would cause a non operational state for a long time due to lack of APIs implemented and the WDDM. Windows 2000 had about 1400 full time devs and 800 full time testers. And ReactOS probably have 10-20 active contributors maybe. I don't know the numbers but there is a huge difference. And I can understand the requirements of an incremental development approach.
I assume it's because of the backward compatibility. But it also would be because of the fact that to implement 5.2 APIs, they need XDDM. Skipping right from 5.2 to 6.2 would cause a non operational state for a long time due to lack of APIs implemented and the WDDM. Windows 2000 had about 1400 full time devs and 800 full time testers. And ReactOS probably have 10-20 active contributors maybe. I don't know the numbers but there is a huge difference. And I can understand the requirements of an incremental development approach.