It isn't any less work. Mapping and scripting aside, it would also need new art for factions\NPC that don't have anything close to it in the base game or the fallout database, like the legion, people on cross, the kings, ect... And that is just taking a superficial look at it, beside, unless someone is doing it for the fun of modding, with no pressure and certainty that it must be finished some day, for fun, I am not sure that NV in F2 engine would interest many people, even on NMA.
Best to try your own completely new thing if you want to try making a total conversion, in any engine. At least you aren't limited in following a established vision, and so you can change things if some technical issues or lack of art skills is bloking you at some point, among other considerations.