UT3 maps are a different beast. maps are just that, Maps
Fallout ESPs are Settings/Maps/references rolled in to one file. they override default game settings, not to mention the majority of the Mods out there use custom Textures / Models. (im sure its the custom model/textures that are the stumbling block, being completely out of the control of Bethesda or MS/Sony)
Yeah, maps are small and contained, even if they do include new artwork. Fallout 3/NV plug-ins can be quite expansive, change things that are already established in the game, and have to integrate themselves into an enormous network of world areas and scripts. This makes them far more complex, which decreases the likelihood that a PC/PS3 converter will work, and it also increases the change of a problem if you can load it.
If plug-ins access is going to be integrated in to these games it has to be directly, not through converters or other hurdles. Bethesda has mentioned wanting to do this with Skyrim if possible but I think it's more likely that we'll see something like this with the next generation consoles.