I don't know why the article frames it as some crazy big mystery and tries to come up with different hypotheses for it throughout. The answer is clear from the very first example they start with, and explained further towards the end – the cash is circulating in the grey/black market economy and across international borders. Demand for cash is rising because this economy is growing at a similar rate to the regular one.
As for the "why does nobody care part", the possible explanation is unconvincing. Yes central banks enjoy the power of being the sole issuer of currency, but the few cents per note they get to keep is hardly where their real interests lie.
Or it could be kind of "propoganda" / PR from the people who are proponents of a cashless society. An assassination of cash in the media would be the logical place to start.
As for the "why does nobody care part", the possible explanation is unconvincing. Yes central banks enjoy the power of being the sole issuer of currency, but the few cents per note they get to keep is hardly where their real interests lie.