Objective C, the syntax, has very little value without Objective C runtime/standard library and Objective C runtime/stdlib is huge (compared to C and C++ standard libraries).
NDK has a very specific goal: if the code would be too slow when compiled to Dalvik VM, you can get speed with NDK, writing in C/C++, but at the cost of loosing access to almost all of the APIs that Android provides. This tradeoff more or less makes sense only for games.
Implementing Objective C runtime and standard library would be huge undertaking and the code, when used, would occupy more memory. Also, this code would mostly duplicate C++ standard library, just with a different syntax.
It doesn't make sense to allow people using Objective C in NDK.