I haven't done any Minetest modding, but I'm sure a pretty large part of that is due to the lack of a JVM. Forge can easily enough unpack the jar for clean enough source to plug into. C++ requires the Minetest devs themselves to guess what modders want to do and build Lua entrypoints. With the new Windows 10/Pocket edition of Minecraft being C++ as well, they'd have to do the same thing I would think.