I can't speak for Lua, but for C, it's because all memory management is handled explicitly by the programmer - that precludes optimizations where dynamic allocation is replaced with stack allocation. The C compiler isn't free to change such things, and most C programmers already perform that optimization without thinking about it. (I don't mean to imply that state of affairs is a good thing, it's just so ingrained in a C programmer that they don't think of it as an optimization.)

Also, since C allows access to raw-memory by design, it can't guarantee that, say, my memory allocation in function f() isn't touched by my funky pointer-arithmetic in function g(). (Perhaps in theory it could, but in practice this problem is mind-blowingly difficult.)

See http://en.wikipedia.org/wiki/Escape_analysis