Because a broken subset of C is much easier to implement as a compiler. Why not use a LLVM/Clang as a compiler backend? Because they didn't exist back in 1423 when Lite-C was invented. But really, it's because with LLVM/Clang we would have never had the drama of the Chinese freelancer that bailed halfway through implementing the compiler leaving JCL with a bunch of Chinese comments and a voodoo black magic compiler.