Why do Central Banks considering implementation of digital currencies whilst some are banning cryptocurrencies ?

There is no absolute answer.

Here are a few possibilities:

1.) The blockchain technology is maturing.
2.) Anti-money laundering and anti-terrorism strategy.
3.) Cryptocurrencies are tax free,whilst CBDC will be taxable.
4.) Natural progression of technology and society.
5.) Empowering centralized governance - more control and traceability by the central government.

Submit your comments on what you think is right.