why Niederreiter cryptosystem is not a candidate in NIST PQC competition?
Classic McEliece is using the Niederreiter T-OWF:

The KEM is built conservatively from a PKE designed for OW-CPA security, namely Niederreiter’s  dual  version  of  McEliece’s  PKE  using  binary  Goppa  codes.

https://classic.mceliece.org/nist/mceliece-20201010.pdf