Sometimes or rather often developers put modability not on the priority list, which makes developing the engine way easier.
BF42 and its engine (includes Vietnam and even BF2/2412, as their engine only is a (major) updated BF42 one) is very special, one of the most modfocused engines floating around. Other engines allow vast editing too (Crysis, Unreal etc.), but never is it possible to mod to such extents with so little difficulty as with BF42.
And I fear the BF43 Frostbite engine will have limitations in these regards, but I'll happily be proven wrong. Especially the "episode concept" and online distribution might be a reason for non modability. Perhaps EA sees it as a danger to the selling rates of possible addons if people start giving out the exact same contents with mods earlier in time or simulatenously...who knows...
The game itself looks nice (nice effects, nice destructibility etc.), but 24 player limitation in BF? Ridiculous. 3 maps to choose from? Ridiculous. Pacific-only? Ridiculous.